Abstract:
A semiconductor structure and a method of forming same is disclosed. The method includes forming, on a substrate, an n-doped collector structure of InAs/AlSb materials; forming a base structure on said collector structure which base structure comprises p-doped GaSb; and forming, on said base structure, an n-doped emitter structure of InAs/AlSb materials. The collector and emitter structure are preferably superlattices each comprising a plurality of periods of InAs and AlSb sublayers. A heterojunction bipolar transistor manufactured using the method is disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains to an InAs/GaSb/AlSb semiconductor structure useful in making bipolar junction transistors (BJTs), and more particularly useful in making heterojunction bipolar transistors (HBTs) and still more particularly useful for making npn HBTs having small (submicron) feature sizes. The present invention also pertains to a method of making same.  
         BACKGROUND  
         [0002]    HBT integrated circuits have found wide acceptance in industry for use in applications as diverse prior art GaAs devices.  
           [0003]    HBTs manufactured from an Indium Arsenide/Aluminum Antimonide/Gallium Antimonide (InAs/AlSb/GaSb) system possess a number of advantages over prior art GaAs HBTs. For example, GaSb is an excellent high-frequency p-type base material, having higher hole mobility than presently used base materials, such as GaAs and In 0.53 Ga 0.47 As. It can be p-doped with Si to densities approaching  10   20 /cm 3 , which are equal to the highest densities achievable with GaAs and InGaAs (using C as a dopant). Since base resistance is inverse to the product of hole mobility and doping level, the higher mobility translates to lower resistance for p-doped GaSb bases, which will increase the operating frequency limit of a device.  
           [0004]    An InAs/AlSb/GaSb material system is also preferable for HBTs being fabricated with submicron feature sizes. Prior art HBTs have relatively low surface Fermi level pinning energy, and as a consequence suffer from recombination of carriers at mesa sidewalls resulting in substantial surface depletion effects. As feature dimensions shrink, these surface effects become proportionally more significant, limiting the size reduction which can be achieved without excessive loss of device performance in terms of the gain of the device. By contrast, the surface pinned energy for GaSb is near the valence band maximum. Accordingly, a p-type GaSb base layer would not have significant surface depletion effects at mesa sidewalls, and could thus be scaled down with less loss of gain due to such surface effects.  
           [0005]    In addition to the above advantages, the InAs/AlSb/GaSb material system allows very flexible bandgap engineering. InAs, AlSb, and GaSb have nearly equal lattice constants, such that varying combinations of the materials may be fabricated, in reasonable thicknesses, without suffering serious crystalline defects. Consequently, flexible engineering is possible which will permit implementation of advanced features, such as drift fields in the base material to sweep minority carriers across the base to the collector.  
           [0006]    A pnp InAs/AlSb/GaSb structure has been tested, as reported by Pekarik et al., “An AlSb-InAs-AlSb double-heterojunction P-n-P bipolar transistor,” J. Vac. Sci. Technol. B, volume 10 no. 2, March/April 1992, pps. 1032-1034. This device does not employ either a GaSb base nor a superlattice in the emitter or collector, and is not of the preferred npn structure. Npn HBT devices are generally preferred by those skilled in the art for high performance applications.  
           [0007]    Although desirable, InAs/AlSb/GaSb heterostructure systems have been difficult to fabricate. First, the available emitter materials, AlGaSb or AlSb, require Tellurium (Te) for n-type doping. Tellurium is inconvenient for use in Molecular Beam Epitaxy (MBE) systems, because its memory effects make it difficult to avoid unwanted Te in subsequent layers, and because the Te ties up an available port. Second, AlGaSb and AlSb have a conduction band mismatch with the preferred GaSb base materials. If they are to be used as emitter materials, they need sophisticated grading to deal satisfactorily with the conduction band offset. They are even less desirable as collector materials, because the noted conduction band offset can cause a trapping of carriers.  
           [0008]    Accordingly, there is a need for an InAs/AlSb/GaSb HBT structure having a GaSb base, and having improved conduction band alignment across the base-emitter and base-collector junctions. Ideally, such an HBT would be easy to dope. The present invention addresses these needs by employing an InAs/AlSb superlattice which can be constructed to achieve nearly perfect conduction band alignment with GaSb. The entire HBT structure can be doped using only Si for both n-doping of the emitter and collector and forp-doping of the GaSb base. Moreover, the InAs/AlSb superlattice has a valence band offset to GaSb of approximately 0.475 V, enhancing the gain characteristics of devices fabricated according to the present invention.  
           [0009]    Si-doped InAs/AlSb superlattices have been used as n-type cladding layers for infrared lasers, as described in U.S. Pat. No. 5,594,750 to Hasenberg and Chow. They have also been used as Schottky barrier layers [Chow, Dunlap, et al., IEEE ElectronDevice Letters, Vol. 17, p. 69 (1996)].  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide a method of forming an InAs/GaSb/AlSb structure which can be used in the manufacture of HBTs which permits conduction-band alignment of the junctions.  
           [0011]    Preferably, the layers of the structure can be easily doped to desired densities and therefore the use of Te as a dopant can be avoided.  
           [0012]    Briefly, and in general terms, the present invention provides a method of forming a semiconductor structure comprising the steps of: (i) forming, on a substrate, an n-doped collector structure of InAs/AlSb materials; forming a base structure on said collector structure which base structure comprises p-doped GaSb; and forming, on said base structure, an n-doped emitter structure of InAs/AlSb materials.  
           [0013]    Preferably the collector and/or emitter structures are provided by superlattice structures having sublayers of InAs and AlSb with thicknesses selected to yield a conduction band edge for the superlattice structures approximately equal to the conduction band edge of GaSb.  
         BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]    [0014]FIG. 1 depicts the conduction and valence band edges of an InAs/AlSb superlattice as a function of constituent layer thickness;.  
           [0015]    [0015]FIG. 2 is a diagram of the band energies for the preferred emitter, base and collector; and  
           [0016]    [0016]FIG. 3 depicts the structure of a semiconductor structure according to the present invention;  
           [0017]    FIGS.  3 A- 3 E depict details of the structure shown by FIG. 3;  
           [0018]    [0018]FIG. 4 depicts the structure of an alternative embodiment of the semiconductor structure of FIG. 3 with a superlattice base; and  
           [0019]    [0019]FIG. 5 depicts how the structures of FIGS.  3  and/or  4  may be etched and metallized in order to provide an HBT device. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    [0020]FIG. 1 is a graph of the conduction and valence band edges of an InAs/AlSb superlattice as a function of constituent layer thicknesses. In this case the InAs sublayer thickness is equal to the AlSb sublayer thickness in each constituent layer (or period) of the superlattice. The conduction band edge of the superlattice can be varied over a wide range, including values above and below the GaSb conduction band (E C   GaSb ) at about 1.2 eV above the valence band maximum of InAs. As can be seen, the conduction band of the superlattice (E C   sls ) will equal the GaSb conduction band when the thickness of the constituent InAs and AlSb sublayers equal about 7.5 Å. This fact will prove useful when selecting the thicknesses of certain InAs and AlSb layers in superlattice structures used in the preferred embodiment of the present invention to be discussed with reference to FIG. 3. In contrast to the conduction band energy, the valence band energy of the superlattice (E v   sls ) does not change significantly with layer thickness and is located 400 meV below the valence band maximum of GaSb (E v   GaSb ).  
         [0021]    [0021]FIG. 2 is a flat band diagram for a HBT structure with a n-type InAs/AlSb superlattice emitter, p-type GaSb base and an n-type InAs/AlSb superlattice collector. The diagram shows both the bulk InAs and AlSb band edges and the effective superlattice band edges in the emitter and collector layers of the HBT device. Preferably, the constituent superlattice layer thicknesses are selected such that the conduction band edges in both the emitter and collector align with the conduction band edge in the GaSb base so that there is a zero conduction band offset, while at the same time the valence band edges are appropriately misaligned by approximately 400 meV. How this can be accomplished will be described with reference to FIG. 3.  
         [0022]    Superlattice structures are welt known in the art. For additional information the reader is directed to “Structural and transport properties of InAs/AlSb superlattices” by D. H. Chow et al. published in the Journal of Crystal Growth vol 150 (1995) at pages 879-882, the disclosure of which is hereby incorporated herein by this reference. The reader is also directed to “InAs/AlSb/GaSb Resonant Interband Tunneling Diodes and Au-on-InAs/AlSb-Superlattice Schottky Diodes for Logic Circuits” by D. H. Chow et al published in IEEE Electron Devices Letters, Vol 17, No. 2, February 1996, the disclosure of which is hereby incorporated hereby by this reference.  
         [0023]    [0023]FIG. 3 shows an epitaxial structure for making a HBT in accordance with the present invention. Substrate  302  is preferably GaSb at the present time, although it is anticipated that continued improvements in compliant substrates may permit the use of other substrate materials, such as GaAs or InP. Subsequent layers are preferably grown by standard Molecular Beam Epitaxy (MBE) techniques, though any technique capable of providing the correct layer structure would be satisfactory. An undoped GaSb buffer layer  304  is grown on substrate  302  to a thickness of about 2000 Å. A subcollector  306 , grown upon buffer layer  304 , is preferably InAs, n-doped to a density of about 10 19 /cm 3  using Si, and is grown to a thickness of about 2000 Å. InAs provides nearly perfect ohmic contact to the non-alloyed metallization, not shown, which will be deposited on subcollector  306  to provide the collector connection. The preferred metallization is Gold Germanium (AuGe), although Au and Al (Aluminum) are also considered to be satisfactory metals for the contacts formed for the emitter, base and collector.  
         [0024]    An optional subcollector grading layer  308  is preferably a chirped superlattice which shifts the effective collector composition gradually to lattice match the InAs preferred for subcollector  306 , thereby reducing charge-impeding band discontinuities between collector  312  and subcollector  306 . Subcollector grading layer  308  includes about ten grading periods  310 , each period  310  having a sublayer  307  of AlSb and a sublayer  309  of InAs. Each period  310  may be conveniently of the same thickness, preferably about 50 Å, but the thicknesses of the periods  310  may range from about 2 to 100 Å and the periods  310  need not necessarily all be of the same thickness. Preferably, n-doping of subcollector grading layer  308  is effected to a desired doping level of about 1*10 19 /cm 3  by doping only InAs-containing sublayers  309  with Si, to a density equal to the desired doping level divided by the proportion of the InAs-containing sublayer  309  within the particular period  310 . Preferably, the thicknesses of the AlSb sublayers  307  increase with the thicknesses of the associated InAs sublayers  309  decreasing as the periods  310  progress from the period immediately adjacent subcollector  306  towards collector  312 .  
         [0025]    For an example, consider the layer structure of grading layer  308  in the preferred embodiment, as shown in FIG. 3 and as shown in even greater detail by FIG. 3A. FIG. 3A shows a small portion of layer  306  and the first two periods immediately adjacent layer  306 , namely the immediately adjacent period  301 - 1  and the next following period  310 - 2 , as well as sublayer  307  of the third period. In the first period  310 - 1  of subcollector grading layer  308 , sublayer  307  comprises a layer of AlSb with a thickness of preferably {fraction (1/22)} of the period  310  thickness (which is preferably 50 Å) so the thickness of sublayer  307  in the first period  310 - 1  is preferably about 2.273 Å thick, while sublayer  309  comprises a layer of InAs having a thickness of {fraction (21/22)} of the period  310  thickness, so the thickness of sublayer  308  in the first period  310 - 1  is preferably about 47.727 Å. The thickness of the first period  310 - 1  is 50 Å, since, as indicated above, 50 Å is the preferred thickness for each period  310 . In each subsequent period  310 , sublayer  307  increases in thickness by about {fraction (1/22)} of the period thickness (or about 2.273 Å) while sublayer  309  decreases in thickness by about the same amount. Thus, the thickness of AlSb sublayer  307  of the tenth period  310 - 10  is preferably {fraction (10/22)} of the period  310  thickness (or about 22.73 Å), while the thickness of the InAs sublayer  309  of the tenth period  310 - 10  is preferably {fraction (12/22)} of the period  310  thickness (or about 27.27 Å). Since the desired average doping density is preferably 2*10 18 /cm 3 , the InAs sublayer  309  in the first period  310 - 1  preferably is doped to (50/47.727)×2*10 18 /cm 3 , or about 2.1*10 18 /cm 3 , while the InAs sublayer  309  in the tenth period preferably is doped to (50/27.27)×2*10 18 /cm 3 , or about 3.67*10 18 /cm 3 .  
         [0026]    This technique results in a constant average doping density through the optional grading layer  308 . However, since the grading layer  308  is itself optional, the grading layer  308 , if used, may be of a more simple construction. For example, the preferred average doping density of 2*10 18 /cm 3  for the grading layer  308  could be maintained by keeping the dopant concentration in each InAs sublayer  309  constant as opposed to adjusting the doping depending on the thickness of each sublayer  309  of InAs.  
         [0027]    Collector  312  is preferably provided by a superlattice of InAs and AlSb, grown to a thickness of preferably 3000 Å. Turning also to FIG. 3B, a small portion (two periods  314 - 1  and  314 - 2  are identified) of the superlattice collector  312  immediately adjacent the tenth period  301 - 10  of the grading layer  308  is depicted. Superlattice periods  314  each have a sublayer  311  of AlSb and a sublayer  313  of InAs. If, within each period  314 ,the thickness of the AlSb sublayer  311  is equal to the thickness of its adjacent InAs sublayer  313 , the resultant superlattice collector  12  is very nearly lattice-matched to GaSb, having a mismatch (Δa/a) of only 5×10 −4 . As has been explained with reference to FIG. 1, the conduction band energy of the superlattice collector  312  is a function of the thicknesses of sublayers  311  and  313 . Sublayer thicknesses of about 7.5 Å for the sublayers  311  and  313  in the periods  314  of the superlattice are preferred (particularly near the base  316 ) in order for the conduction band energy at the edge of the collector superlattice  312  to align with the conduction band of the base  316  of the HBT, which base  316 , as will be seen, is preferably formed of doped GaSb. In order to achieve an effective average doping density of about 10 16 /cm 3 , the AlSb sublayers  311  are preferably undoped while the InAs sublayers  313  are preferably doped to about 2*10 16 /cm 3 . The overall thickness of the collector  312  is preferably 3000 Å. The thickness of each period  314  is preferably 15 Å, at least adjacent the base  316 . If the 15 Å period  314  thickness were maintained throughout the entire preferred 3000 Å thickness of the collector  312 , then the collector  312  would comprise approximately 200 periods  314 . Preferably, however, the periods  314  increase slightly in thickness as the periods are more and more remote from the base, so that the period  314  thickness preferably increases to about 20 Å in the center of the base  316 .  
         [0028]    In the embodiment of FIG. 3, base  316  is preferably bulk GaSb grown to a thickness of preferably 300 Å. The GaSb base is p-doped preferably with Si to a density of about 10 20 /cm 3 .  
         [0029]    Emitter  322 , grown upon base  316 , is preferably a superlattice having repeating periods  324 , each period  324  including a sublayer  323  of AlSb and a sublayer  321  of InAs. FIG. 3C shows two periods  324 - 1  and  324 - 2 , period  324 - 1  being the period  324  immediately adjacent base  316 . It is generally preferred to align the conduction band energies of both the base-emitter and base-collector junctions. In this preferred embodiment, since base  316  is bulk GaSb, emitter  322  is preferably grown in a structure similar to that of collector  312  in order to attain the same conduction band energy of about 1.2 eV above the valence band maximum of InAs. Accordingly, sublayers  323  and  321  are each preferably 7.5 Å thick. Emitter  322  differs from collector  312  in that it is preferably grown to a lesser thickness (of about 1000 Å) and is doped more heavily than the collector  312 . Emitter  312  is preferably n-doped, using Si, to a density of about 10 18 /cm 3 , by doping the InAs sublayers  321  to a density of 2*10 18 /cm 3 . In that way the AlSb sublayers  323  may be left undoped.  
         [0030]    Emitter contact grading layer  318  is somewhat similar to the subcollector grading layer  308  in this embodiment, the emitter contact grading layer  318  including preferably a number of periods  320 , starting with a period thickness of about 15 Å at the juncture with the emitter layer  322  and increasing to a period thickness of about 50 Å thick adjacent the InAs contact layer  330 . Each period  320  comprises an InAs sublayer  319  and a AlSb sublayer  317 . FIG. 3D is a detailed view of a first few periods  320 - 1  through  320 - 3  of the emitter contact grading layer  318  immediately adjacent the last two periods  324 -X and  324 -X- 1  of the emitter  322 . If the total thickness of the emitter is indeed about 1000 Å, then X (the number of periods  324  in the superlattice emitter  322 ) will fall in the range of about 65 to 70. The proportional thickness of each InAs sublayer  319  within its period  320  conveniently increases each period from about {fraction (11/22)} in the first period  320 - 1  nearest emitter  322  to about {fraction (21/22)} in the last period  320 -X immediately adjacent emitter contact  330 . FIG. 3E is a detailed view of the last two periods  310 -X- 1  and  310 -X next to the emitter contact  330 . The thickness of each of the periods  320  is conveniently increased to about 50 Å while decreasing the proportional thickness of each AlSb sublayer  317  from about {fraction (11/22)} of the first period  320 - 1  to about {fraction (1/22)} of the last period  320 -X. Emitter contact grading layer  318  has an overall preferred thickness of about 1000 Å. Doping is accomplished as in subcollector grading layer  308 , by using Si n-type doping of the InAs sublayers  319  to achieve an average doping density of preferably 2*10 18 /cm 3  in emitter contact grading layer  318 .  
         [0031]    In the preferred embodiment of FIG. 3, emitter contact layer  330  is preferably bulk InAs grown to a thickness of preferably 300 Å. The InAs emitter contact  330  is n-doped preferably with Si to a density of about 10 19 /cm 3 .  
         [0032]    In the embodiment of FIG. 3, the base layer is described as being provided by bulk GaSb grown using known epitaxial techniques, such as MBE. However, instead of using a bulk material for the base, if a graded AlGa Sb alloy is provided instead, then a drift field for electrons crossing the base layer  316  can be provided and the emitter superlattice energy gap can be widened slightly to provide conduction band edge alignment at both the emitter-base and collector-base junctions. For example, Al 0.2 Ga 0.8  Sb has a higher conduction band maximum than does GASb by about 100 meV. As such the InAs/AlSb superlattice emitter period  324  would have a smaller thickness in order to provide the appropriate alignment with the E C  for Al 0.2 Ga 0.8  Sb.  
         [0033]    [0033]FIG. 4 shows another alternative embodiment of the base  316 . Here the base  316  is a chirped superlattice, graded in ten periods  317  from GaSb at the collector junction to Al x Ga 1-x Sb at the emitter junction. The Al x Ga 1-x Sb composition may have x at the emitter end of from 0 to about 0.2. However, x is preferably equal to 0.1 and thus the base material  317  adjacent the emitter is preferably formed by Al 0.1 Ga 0.9 Sb.  
         [0034]    The epitaxially grown structures shown in FIG. 3 or  4 , after suitable masking and etching using well known techniques, can be etched to obtain the shape of the structure shown in FIG. 5. With the application of suitable metalization, the HBT structure shown in FIG. 5 is obtained. The metalization includes forming collector contacts  340 , base contacts  345  and emitter contacts  350 . Since masking and etching using the structures shown in FIG. 3 or  4  to attain the structure shown in FIG. 5 is rather straightforward for those skilled in the art, the details for how the masking and etching is carried out is a matter of design choice.  
         [0035]    Having described the invention in connection with its preferred embodiments, modification will now suggest itself to those skilled in the art. As such the invention is not to be limited to the disclosed embodiments, expect as required by the appended claims.