Semiconductor device having improved buffer layers

Abstract

A first buffer layer is formed on a substrate at a lower temperature than a single-crystal-growth-temperature, one or more of a layer composed of a nitride containing neither Ga nor In, a layer which has two or more thin films having different moduli of elasticity cyclically laminated therein, and a layer having an Al composition ratio which decreases and a Ga composition ratio which increases in a direction from the first buffer layer to a device-constituting layer are formed as a second buffer layer on the first buffer layer at the single-crystal-growth-temperature, and a device-constituting layer composed of a nitride semiconductor is formed on the second buffer layer.

What is claimed is:  
     
         1 . A semiconductor device comprising in the following order: 
 a substrate;    a first buffer layer in a non-single crystalline state;    a second buffer layer in an approximately single crystalline state composed of a nitride containing neither Ga nor In; and    a device-constituting layer composed of a nitride semiconductor.    
     
     
         2 . The semiconductor device according to  claim 1 , wherein 
 said second buffer layer is composed of Al 1-x B x N (0≦x≦1).    
     
     
         3 . The semiconductor device according to  claim 2 , wherein 
 said first buffer layer is formed of a single layer film or a multi-layer film composed of a nitride containing at least one of Al, Ga, In, B and Tl, SiC or ZnO.    
     
     
         4 . A semiconductor device comprising in the following order: 
 a substrate;    a first buffer layer in a non-single crystalline state;    a second buffer layer in an approximately single crystalline state which has two or more thin films having different moduli of elasticity cyclically laminated therein; and    a device-constituting layer composed of a nitride semiconductor.    
     
     
         5 . The semiconductor device according to  claim 4 , wherein 
 said second buffer layer comprises first nitride films and second nitride films which are alternately laminated,    said first nitride film has an Al composition ratio higher than that of said second nitride film, and    said second nitride film has a Ga composition ratio higher than that of said first nitride film.    
     
     
         6 . The semiconductor device according to  claim 5 , wherein 
 the thickness of said first nitride film gradually decreases in a direction from said first buffer layer to said device-constituting layer, and    the thickness of said second nitride film gradually increases in a direction from said first buffer layer to said device-constituting layer.    
     
     
         7 . The semiconductor device according to  claim 5 , wherein 
 said first nitride film and said second nitride film are formed in this order on said first buffer layer.    
     
     
         8 . The semiconductor device according to  claim 5 , wherein 
 said first nitride film is composed of AlN, and    said second nitride film is composed of GaN.    
     
     
         9 . The semiconductor device according to  claim 4 , wherein 
 said first buffer layer is formed of a single layer film or a multi-layer film composed of a nitride containing at least one of Al, Ga, In, B and Tl, SiC or ZnO.    
     
     
         10 . A semiconductor device comprising in the following order: 
 a substrate;    a first buffer layer in a non-single crystalline state;    a second buffer layer in an approximately single crystalline state; and    a device-constituting layer composed of a nitride semiconductor,    said second buffer layer having an Al composition ratio which decreases and a Ga composition ratio which increases in a direction from said first buffer layer to said device-constituting layer.    
     
     
         11 . The semiconductor device according to  claim 10 , wherein 
 said first buffer layer is formed of a single layer film or a multi-layer film composed of a nitride containing at least one of Al, Ga, In, B and Tl, SiC or ZnO.    
     
     
         12 . A semiconductor device comprising in the following order: 
 a substrate;    a first buffer layer in a non-single crystalline state;    a second buffer layer in an approximately single crystalline state; and    a device-constituting layer composed of a nitride semiconductor,    said second buffer layer comprising two or more layers selected from a layer composed of a nitride containing neither Ga nor In, a layer which has two or more thin films having different moduli of elasticity cyclically laminated therein, and a layer having an Al composition ratio which decrease and a Ga composition ratio which increases in a direction from said first buffer layer to said device-constituting layer.    
     
     
         13 . The semiconductor device according to  claim 12 , wherein 
 said second buffer layer comprises among said two or more layers an intermediate layer having a lattice constant closer to the lattice constant of said device-constituting layer than the layer, on the side of said first buffer layer, out of said two or more layers.    
     
     
         14 . The semiconductor device according to  claim 12 , wherein 
 said first buffer layer is formed of a single layer film or a multi-layer film composed of a nitride containing at least one of Al, Ga, In, B and Tl, SiC or ZnO.    
     
     
         15 . A method of fabricating a semiconductor device, comprising the steps of: 
 forming a first buffer layer on a substrate at a lower temperature than a single-crystal-growth-temperature;    forming on said first buffer layer a second buffer layer composed of a nitride containing neither Ga nor In at the single-crystal-growth-temperature; and    forming a device-constituting layer composed of a nitride semiconductor on said second buffer layer.    
     
     
         16 . The method according to  claim 15 , wherein the step of forming said second buffer layer comprises the step of forming a layer composed of Al 1-x B x N (0≦x≦1).  
     
     
         17 . A method of fabricating a semiconductor device, comprising the steps of: 
 forming a first buffer layer on a substrate at a lower temperature than a single-crystal-growth-temperature;    forming on said first buffer layer a second buffer layer which has two or more thin films having different moduli of elasticity cyclically laminated therein at the single-crystal-growth-temperature; and    forming a device-constituting layer composed of a nitride semiconductor on said second buffer layer.    
     
     
         18 . The method according to  claim 17 , wherein 
 the step of forming said second buffer layer comprises the step of alternately laminating first nitride films and second nitride films such that the first nitride film has an Al composition ratio higher than that of the second nitride film, and said second nitride film has a Ga composition ratio higher than that of said first nitride film.    
     
     
         19 . The method according to  claim 18 , wherein the step of forming said second buffer layer comprises the step of alternately laminating said first nitride films and said second nitride films such that the thickness of said first nitride film gradually decreases in a direction from said first buffer layer to said device-constituting layer, and the thickness of said second nitride film gradually increases in a direction from said first buffer layer to said device-constituting layer.  
     
     
         20 . The method according to  claim 18 , wherein the step of forming said second buffer layer comprises the step of forming an AlN film as said first nitride film, and forming a GaN film as said second nitride film.  
     
     
         21 . A method of fabricating a semiconductor device, comprising the steps of: 
 forming a first buffer layer on a substrate at a lower temperature than a single-crystal-growth-temperature;    forming a second buffer layer on said first buffer layer at the single-crystal-growth-temperature; and    forming a device-constituting layer composed of a nitride semiconductor on said second buffer layer,    the step of forming said second buffer layer comprising the step of forming a layer having an Al composition ratio which decreases and a Ga composition ratio which increases in a direction from said first buffer layer to said device-constituting layer.    
     
     
         22 . A method of fabricating a semiconductor device, comprising the steps of: 
 forming a first buffer layer on a substrate at a lower temperature than a single-crystal-growth-temperature;    forming a second buffer layer on said first buffer layer at the single-crystal-growth-temperature; and    forming a device-constituting layer composed of a nitride semiconductor on said second buffer layer,    the step of forming said second buffer layer comprising the step of forming two or more layers selected from a layer composed of a nitride containing neither Ga nor In, a layer which has two or more thin films having different moduli of elasticity cyclically laminated therein, and a layer having an Al composition ratio which decreases and a Ga composition ratio which increases in a direction from said first buffer layer to said device-constituting layer.