Method for reducing heterostructure acoustic charge transport device SAW drive power requirements

Abstract

A HACT device employing a thin-film overlay of a more strongly piezoelectric material can operate as a delay line and as a tapped delay line, or transversal filter, while requiring less total power for the SAW clock signal. The increased electrical potential per unit total SAW power thus realized facilitates coupling between the total SAW energy and the mobile charge carriers. Some materials systems, such as a GaAs substrate and a ZnO thin-film overlay, will require an intervening thin-film dielectric layer in between the HACT substrate and epitaxial layers and the thin-film piezoelectric overlay. This may be necessitated by chemical, semiconductor device processing, or adhesion incompatibilities between the substrate material and the thin-film overlay material.

What is claimed is: 
     
       1. A method for increasing the charge carrying capacity of a heterostructure acoustic charge transport device, comprising the steps of: providing a semiconductor substrate having a surface;   fabricating an interdigital metallic surface acoustic wave transducer along said surface of said semiconductor substrate;   providing a channel including epitaxial layers of semiconductor material along said surface of said semiconductor substrate; .Iadd.and .Iaddend.   depositing a . .thin-film.!. layer of piezoelectric material along said channel of said semiconductor substrate . .said thin-layer of dielectric material being more strongly dielectric than said semiconductor substrate;   providing electric charge to said channel of said substrate; and   transporting said electric charge through said channel..!..Iadd., wherein said layer of piezoelectric material is more strongly piezoelectric than said semiconductor substrate. .Iaddend.   
     
     
       2. A method for increasing the charge carrying capacity for a heterostructure acoustic charge transport device as claimed in claim 1, wherein there is further included the step of interposing a . .thin-film.!. dielectric layer between said channel and said piezoelectric . .thin-film.!. layer, said . .thin-film.!. dielectric layer for promoting adhesion-of said piezoelectric . .thin-film.!. layer to said channel. 
     
     
       3. A method for increasing the charge carrying capacity of a heterostructure acoustic charge transport device as claimed in claim 1, wherein there is further included the step of attaching an acoustic absorber means . .attached.!. to said surface of said semiconductor substrate to prevent reflection of said surface acoustic wave. 
     
     
       4. A method for increasing the charge carrying capacity of a heterostructure acoustic charge transport device as claimed in claim 1, wherein there is further included the steps of: providing an input ohmic contact for coupling said electric charge to said channel;   providing an input Schottky contact for controlling the transportation of said electric charge through said channel,   providing a Schottky nondestructive sensing electrode array coupled to said channel for monitoring said transported electric charge;   providing an output Schottky contact for controlling the flow of electric charge through said channel; and   providing an output contact for receiving a replica of said input electric charge.   
     
     
       5. A method for reducing drive power requirements of a SAW transducer in a heterostructure acoustic charge transport device, comprising the steps of: providing a semiconductor substrate having a surface;   fabricating an interdigital metallic surface acoustic wave transducer along said surface of said semiconductor substrate;   providing a channel including epitaxial layers of semiconductor material along said surface of said semiconductor substrate; .Iadd.and .Iaddend.   depositing a . .thin-film.!. layer of piezoelectric material along said channel of said semiconductor substrate . .;   providing electric charge to said channel of said substrate; and   transporting said electric charge through said channel with approximately 50 milliWatts of power applied to said surface wave transducer..!. .Iadd., wherein said layer of piezoelectric material is more strongly piezoelectric than said semiconductor substrate. .Iaddend. .Iadd.   
     
     
       6.  A method for increasing the charge carrying capacity of a heterostructure acoustic charge transport device, said method comprising steps of: providing a semiconductor substrate having a surface;   disposing an interdigital metallic surface acoustic wave transducer on the surface of the semiconductor substrate;   providing a channel including epitaxial layers of semiconductor material along a <110> axis of the surface of the semiconductor substrate; and   depositing a layer of piezoelectric material along the channel of the semiconductor substrate, wherein the layer of piezoelectric material is more strongly piezoelectric than the semiconductor substrate. .Iaddend. .Iadd.7. A method as claimed in claim 6, wherein said depositing step includes a step of depositing a layer consisting of zinc oxide. .Iaddend. .Iadd.8. A method as claimed in claim 7, wherein said step of depositing the layer of zinc oxide includes a step of depositing a layer consisting of zinc oxide having a thickness of about 0.15 acoustic wavelengths. .Iaddend. .Iadd.9. A method as claimed in claim 6, further including a step of interposing a layer comprising silicon nitride between the channel and the layer of piezoelectric material. .Iaddend. .Iadd.10. A method as claimed in claim 6, wherein said step of providing the semiconductor substrate includes a step of providing a semiconductor substrate comprising gallium arsenide and wherein the surface is a {100}-oriented surface. .Iaddend. .Iadd.11. A method as claimed in claim 6, wherein said depositing step includes a step of depositing a layer of piezoelectric material wherein the layer of piezoelectric material is selected from a group consisting of aluminum nitride, zinc oxide and lithium niobate. .Iaddend. .Iadd.12. A method as claimed in claim 6, wherein there is further included a step of interposing a dielectric layer between the channel and the layer of piezoelectric material, the dielectric layer for promoting adhesion of the layer of piezoelectric material to the channel. .Iaddend. .Iadd.13. A method as claimed in claim 6, wherein said depositing step includes a step of depositing a layer of piezoelectric material consisting of aluminum nitride. .Iaddend.