Abstract:
A metamorphic high electron mobility transistor having a plurality of high electron mobility transistor layers, a semi-insulating substrate, a ternary metamorphic buffer layer positioned between the semi-insulating substrate and the plurality of high electron mobility transistor layers, the ternary metamorphic buffer layer being Al 1-x Ga x Sb such that x is greater than or equal to 0.2 but less than 0.3, a stabilizing layer positioned between the ternary metamorphic buffer layer and the plurality of high electron mobility transistor layers, the stabilizing layer being Al 1-y Ga y Sb such that y is greater than 0.2 but less than or equal to 0.3 and y is greater than x, and a nucleation layer interposed between the semi-insulating substrate and the ternary metamorphic buffer layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     The invention relates generally to High Electron Mobility Transistors (“HEMTs”). More particularly, the invention relates to epitaxial nucleation and buffer sequence for via-compatible InAs/AlGaSb HEMTs. 
     2. Description of Related Art 
     When electrons from ionized donors are placed in a material with higher conduction band energy than the channel, and close to the channel, they create an electron gas with high electron mobility. Transistors that implement this concept are known in the art as High Electron Mobility Transistors (“HEMT”). 
     A HEMT is a field effect transistor with a junction between two materials of different band gaps, the barrier material and the channel material. The barrier material contains donor electrons with higher conduction band energy than the channel material. This improves the electron mobility by separating the donor ions from the conducting channel. The channel material makes up the conducting layer and is selected based on the transport properties of the electrons, with the band gap also being a consideration to support high fields and high voltages. Conventionally, a HEMT system is made from Galium Arsenide (GaAs) with Aluminum Galium Arsenide (AlGaAs). The donor-containing wide bandgap material is AlGaAs and the conducting channel is GaAs. The effect of the junction between these two materials is to create a very thin layer where the Fermi energy is above the conduction band, giving the channel very low resistance and high electron mobility. 
     HEMTs can be used for Monolithic Microwave Integrated Circuits (MMICs). In most cases, MMICs require a semi-insulating substrate to allow the use of microstrip transmission lines and high-Q passive elements such as integrated inductors. MMICs also require a semi-insulating substrate to reduce the substrate loss at high frequencies. 
     Ordinarily, the substrate material and the HEMT materials have the same lattice constant. If the two materials have a different lattice constant, a buffer layer can be placed between them to form a metamorphic HEMT (“mHEMT”). Since InAs-channel HEMT circuits have no lattice-matched substrate that is semi-insulating, all practical InAs-based HEMTs have been grown via metamorphic growth on a semi-insulating substrate with a different lattice constant, usually GaAs. 
     Prior art metamorphic buffers have had thick layers (&gt;1 micrometer) of Aluminum Antimonide (AlSb) or Gallium Antimonide (GaSb). Since a GaSb buffer is too conductive for RF circuits, prior art InAs/AlGaSb HEMTs have typically been grown using an AlSb metamorphic buffer layer. The use of pure AlSb for the majority of the metamorphic buffer presents a problem in integrated circuit fabrication because pure AlSb is very unstable and is prone to oxidation and subsequent cracking of the epitaxial material after AlSb has been exposed to chemicals, such as acid, base, cleaning solvents, water, and even air after several hours of exposure.  FIG. 1  is a cross-sectional photo obtained using a secondary electron microscope (SEM) and illustrates the oxidation of the AlSb buffer. 
     There are several integral steps in MMIC production that expose the AlSb metamorphic buffer to chemicals, including the etching of vias connecting the front and back sides of the wafer and the cleaving of the wafer into individual chips which expose the AlSb at the sidewalls. With an increasing demand for improved MMICs, there remains a continuing need in the art for an epitaxial nucleation and buffer layer sequence that is stable when exposed to chemicals. 
     SUMMARY OF THE INVENTION 
     A metamorphic high electron mobility transistor with reduced threading dislocations and improved chemical stability having a plurality of high electron mobility transistor layers, a semi-insulating substrate, a first ternary metamorphic buffer, and a first nucleation layer. The nucleation layer is between the semi-insulating substrate and the first ternary metamorphic buffer, and the first ternary metamorphic buffer is between the first nucleation layer and the high electron mobility transistor layers. 
     In one embodiment, the metamorphic high electron mobility transistor has a stabilizing layer between the first ternary metamorphic buffer and the high electron mobility transistor layers. The ternary metamorphic buffer is Al 0.8 Ga 0.2 Sb and the stabilizing layer is Al 0.7 Ga 0.3 Sb. In another embodiment, the ternary metamorphic buffer and stabilizing layers are each composed of Al 1-x Ga x Sb, where x is greater than or equal to 0.2 but less than or equal to 0.3. 
     In yet another embodiment, the metamorphic high electron mobility transistor has a second nucleation layer between the first nucleation layer and the first ternary metamorphic buffer, and a third nucleation layer between the second nucleation layer and the ternary metamorphic buffer. The first nucleation layer is GaAs, the second nucleation layer is AlAs, and the third nucleation layer is AlSb. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The exact nature of this invention, as well as the objects and advantages thereof, will become readily apparent from consideration of the following specification in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
         FIG. 1  is a cross-sectional photo obtained using a secondary electron microscope (SEM) and illustrates the oxidation of an AlSb buffer. 
         FIG. 2  is a cross-sectional view of a metamorphic HEMT, according to one embodiment of the invention. 
         FIG. 3  is an exploded view of the metamorphic HEMT of  FIG. 2 , illustrating a nucleation layer between a substrate wafer and a ternary metamorphic buffer layer, according to one embodiment of the invention. 
         FIG. 4  is cross-sectional view of a metamorphic HEMT illustrating a plurality of nucleation layers and a plurality of ternary metamorphic buffer layers, according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  is a cross-sectional view of a metamorphic HEMT (“mHEMT”), according to one embodiment of the invention. The mHEMT has a plurality of HEMT layers  11 , a stabilizing layer  12 , a metamorphic buffer layer  13 , a nucleation layer  15 , and a substrate wafer  17 . The HEMT layers  11  include a metal layer  19 , a first layer of barrier material  21 , a layer of channel material  23 , and a second layer of barrier material  25 . 
     The metal layer  19  can be selectively deposited or removed to form a source (not shown), a drain (not shown) and a gate (not shown). The metal layer can have a thickness of about 0.1 to about 1 μm. 
     The barrier material  21  and  25  can be an Antimonide-based semiconductor, such as Aluminum Antimonide (AlSb), Gallium Antimonide (GaSb), or their respective alloys. AlSb barrier  21  and  25  has about 1.3 eV conduction band offset allowing for sheet carrier concentrations up to 1×10 13  cm −2 . Both AlSb and AlGaSb are stable compounds with insulating properties, and therefore, can be used to isolate the conductive channel material  23 . The barrier layers  21  and  25  can have a thickness of about 150 Å. 
     The channel material  23  can be an Indium-based semiconductor, such as Indium Galium Arsenide (InGaAs), Indium Arsenide (InAs), or their respective alloys. InGaAs channels have electron mobilities of about 10-15,000 cm 2 /Vs for the highest indium concentrations. The binary InAs channel offers the highest room temperature mobility with values of about 30,000 cm 2 /Vs possible for 1×10 −12  cm −2  electron sheet concentrations. The channel layer  23  can have a thickness of about 150 Å. 
     The metamorphic buffer layer  13  is used to buffer or separate two materials of different lattice constants. The composition of the buffer layer  13  depends on the composition of the substrate  17  and/or the barrier layer  25 . For example, if the substrate  17  is GaAs and the barrier layer  25  is AlSb, then the buffer  13  can have a compound with an atomic spacing that allows a smooth transition between the two materials. In this instance, since GaAs and AlSb have about the same atomic spacing, an AlSb metamorphic buffer  13  can be used. Another example, if the substrate  17  is Indium Phosphide (InP) and the barrier layer  25  is GaSb, then the metamorphic buffer  13  can be Indium Aluminum Arsenide (In 50% A 50% As). The metamorphic buffer layer  13  can have a thickness of about 1-2 μm. 
     The substrate wafer  17  can be a semi-insulating substrate, such as GaAs, InP, GaSb, or their respective alloys. The substrate wafer  17  can be about 2 to 6 inches in diameter, but preferably, about 3 to 4 inches in diameter. The substrate wafer  17  can also have a thickness ranging from 250 to 625 μm. 
     If the lattice constant of the substrate  17  is different from the lattice constant of the metamorphic buffer layer  13 , threading dislocations in the metamorphic buffer layer  13  may occur. The number of defects is typically about 10 8  cm −2 . These defects affect the proper functioning of the mHEMT device. 
     Pure AlSb metamorphic buffer  13  presents a problem in integrated circuit fabrication because pure AlSb is very unstable and is prone to oxidation and subsequent cracking of the epitaxial material after AlSb has been exposed to chemicals, such as acids, base, cleaning solvents, and water. AlSb metamorphic buffer  13  can be exposed to chemicals by the etching of vias (not shown) connecting the front  27  and back  29  sides of the wafer, and by the cleaving of the wafer into individual chips which expose the AlSb at the sidewalls  31  and  33 . 
     To reduce threading dislocations and improve chemical stability, a metamorphic nucleation and buffer layer sequence is proposed herein. According to one embodiment of the invention, the ternary metamorphic buffer  13 , such as Aluminum Gallium Antimonide (AlGaSb), can be used instead of or with the pure binary compound, AlSb. 
     Depending on the composition levels of the ternary AlGaSb buffer  13 , the chemical stability and electrical conductivity of the buffer  13  may vary. If the gallium composition is too low, the chemical stability of the alloy will not be sufficient, but if the gallium composition is too high, the resistivity of the metamorphic buffer layer decreases, which increases the high frequency loss of the overlying circuit components. The gallium composition can range from about 5% to about 75% of the alloy by cation atomic fractional composition. However, the optimal alloy composition is preferably between about 20% to about 30% gallium. 
     In one embodiment, the composition of aluminum in the ternary metamorphic buffer  13  depends on the relation Al 1-x Ga x Sb. Hence, if the optimal gallium composition is between about 20% to about 30%, then the optimal aluminum composition is between about 70% to about 80%. 
     In one embodiment, a nucleation layer  15  is grown on the substrate  17  to transition the crystal lattice constant between the substrate  17  and the ternary metamorphic buffer  13 . In another embodiment, a plurality of nucleation layers  15  are used to transition the crystal lattice constant between the substrate  17  and the ternary metamorphic buffer  13 . 
       FIG. 3  is an exploded view of the mHEMT of  FIG. 2 , illustrating the nucleation layer  15  used to transition between the substrate  17  and the metamorphic buffer  13 . The nucleation layer  15 , shown in  FIG. 3 , can have the same composition as the substrate  17  so as to provide a smooth platform for application of the ternary metamorphic buffer  13 . Hence, if the substrate  17  is GaSb, the nucleation layer  15  is also GaSb with a thickness of about 1 μm, but preferably, 50 nm or less. 
     In one embodiment of the invention, three nucleation layers  15  can be used to transition between the ternary metamorphic buffer  13  and the substrate  17 .  FIG. 4  is cross-sectional view of a metamorphic HEMT illustrating a plurality of nucleation layers  41 ,  43  and  45 , according to one embodiment of the invention. If the substrate  17  is GaAs, then the first nucleation layer  41  grown on top of the substrate  17  to smoothen the surface can be GaAs, the second nucleation layer  43  grown on top of the first transition layer  41  can be Aluminum Arsenide (AlAs), and the final nucleation layer  45  grown on top of the second transition layer  43  can be AlSb. The thickness of the GaAs first nucleation layer  41  is preferably about 50 nm or less, the AlAs second nucleation layer  43  is preferably about 10 nm or less, the AlSb third nucleation layer  45  is preferably about 30 nm or less. Those skilled in the art will appreciate that more nucleation layers can be used without departing from the scope and spirit of the invention. 
     Pure AlSb can be used as a final nucleation layer  45  because, unlike other semiconductors, will recover planar growth conditions quickly after the transition in lattice constant. The AlSb nucleation layer  45  can have a thickness of about 1 μm. However, for the purpose of re-planarizing the MBE growth, the AlSb nucleation layer  45  can have a thickness of about 30 nm or less. This layer structure offers much improved chemical stability when the semiconductor layers are etched through, as in back-side vias. 
     As shown in  FIG. 4 , the ternary metamorphic buffer layer  13  and the stabilizing layer  12  are grown on the nucleation layers  41 ,  43 , and  45 . This allows the selection of two layers with different compositions having desirable characteristics. For example, the ternary metamorphic buffer layer  13  can have a composition of Al 0.8 Ga 0.2 Sb with thickness of about 1 μm, while the stabilizing layer  12  can have a composition of Al 0.7 Ga 0.3 Sb with a thickness of about 0.3 μm. Since the stabilizing layer  12  has a higher gallium composition than the ternary metamorphic buffer layer  13 , the stabilizing layer  12  is more stable when exposed to chemicals but also more conductive. To increase the resistivity of the metamorphic buffer layer, a lower gallium composition is used for the ternary metamorphic buffer layer  13 . 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other changes, combinations, omissions, modifications and substitutions, in addition to those set forth in the above paragraphs, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the just described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.