Semiconductor device having epitaxial structures

Abstract

A semiconductor device having epitaxial structures includes a gate structure positioned on a substrate, epitaxial structures formed in the substrate at two sides of the gate structure, and an undoped cap layer formed on the epitaxial structures. The epitaxial structures include a dopant. The epitaxial structures and the undoped cap layer include a first semiconductor material having a first lattice constant and a second semiconductor material having a second lattice constant. The second lattice constant is larger than the first lattice constant. The second semiconductor material in the epitaxial structure includes a first concentration and the second semiconductor material in the undoped cap layer includes a second concentration. The second concentration is lower than the first concentration, and is upwardly decreased.

What is claimed is: 
     
         1 . A semiconductor device having epitaxial structures comprising:
 a gate structure positioned on a substrate;   epitaxial structures formed in the substrate at two sides of the gate structure, the epitaxial structure comprising a dopant, a first semiconductor material having a first lattice constant, and a second semiconductor material having a second lattice constant, the second lattice constant being larger than the first lattice constant, and the second semiconductor material in the epitaxial structure having a first concentration; and   an undoped cap layer formed on the epitaxial structures, the undoped cap layer comprising the first semiconductor material and the second semiconductor material, the second semiconductor material in the undoped cap layer having a second concentration, wherein the second concentration is lower than the first concentration and upwardly decreased.   
     
     
         2 . The semiconductor device having epitaxial structures according to  claim 1 , further comprising:
 lightly-doped drains (LDDs) formed in the substrate respectively at the two sides of the gate structure; and   a spacer formed on sidewalls of the gate structure.   
     
     
         3 . The semiconductor device having epitaxial structures according to  claim 1 , wherein the first semiconductor material comprises silicon (Si) and the second semiconductor material comprises germanium (Ge). 
     
     
         4 . The semiconductor device having epitaxial structures according to  claim 1 , wherein the second concentration is about 25%. 
     
     
         5 . The semiconductor device having epitaxial structures according to  claim 4 , wherein the second concentration is upwardly decreased to 0%. 
     
     
         6 . The semiconductor device having epitaxial structures according to  claim 1 , further comprising an undoped under layer positioned between the epitaxial structure and the substrate, the undoped under layer comprises the first semiconductor material and the second semiconductor material. 
     
     
         7 . The semiconductor device having epitaxial structures according to  claim 6 , wherein a concentration of the second semiconductor material in the undoped under layer is between 10% and 25%. 
     
     
         8 . The semiconductor device having epitaxial structures according to  claim 7 , wherein the concentration of the second semiconductor material in the undoped under layer is upwardly increased. 
     
     
         9 . A semiconductor device having epitaxial structures comprising:
 a gate structure positioned on a substrate;   epitaxial structures formed in the substrate at two sides of the gate structure, the epitaxial structure comprising a dopant, a first semiconductor material having a first lattice constant, and a second semiconductor material having a second lattice constant, the second lattice constant being larger than the first lattice constant, and the second semiconductor material in the epitaxial structure having a first concentration;   a first undoped cap layer formed on the epitaxial structures, the first undoped cap layer comprising the first semiconductor material and the second semiconductor material, the second semiconductor material in the first undoped cap layer having a second concentration, wherein the second concentration is lower than the first concentration and upwardly decreased; and   a second undoped cap layer formed on the first undoped cap layer, the second undoped cap layer comprising the first semiconductor material and the second semiconductor material, the second semiconductor material in the second undoped layer having a third concentration, and the third concentration is equal to 0%.   
     
     
         10 . The semiconductor device having epitaxial structures according to  claim 9 , further comprising:
 lightly-doped drains (LDDs) formed in the substrate respectively at the two sides of the gate structure; and   a spacer formed on sidewalls of the gate structure.   
     
     
         11 . The semiconductor device having epitaxial structures according to  claim 9 , wherein the first semiconductor material comprises silicon (Si) and the second semiconductor material comprises germanium (Ge). 
     
     
         12 . The semiconductor device having epitaxial structures according to  claim 9 , wherein the second concentration is about 25%. 
     
     
         13 . The semiconductor device having epitaxial structures according to  claim 9 , wherein a thickness of the first undoped cap layer and a thickness of the second undoped cap layer have a ratio, and the ratio is about 1:2. 
     
     
         14 . The semiconductor device having epitaxial structures according to  claim 9 , further comprising a third undoped cap layer formed between the first undoped cap layer and the second undoped cap layer, the second semiconductor material in the third undoped cap layer has a fourth concentration, and the fourth concentration is between the second concentration and the third concentration. 
     
     
         15 . The semiconductor device having epitaxial structures according to  claim 14 , wherein the fourth concentration is about 10%. 
     
     
         16 . The semiconductor device having epitaxial structures according to  claim 15 , wherein a thickness of the first undoped cap layer, a thickness of the second undoped cap layer, and a thickness of the third undoped cap layer have a ratio, and the ratio is about 1:1:1. 
     
     
         17 . The semiconductor device having epitaxial structures according to  claim 9 , further comprising an undoped under layer positioned between the epitaxial structure and the substrate, the undoped under layer comprises the first semiconductor material and the second semiconductor material. 
     
     
         18 . The semiconductor device having epitaxial structures according to  claim 17 , wherein a concentration of the second semiconductor material in the undoped under layer is between 10% and 25%. 
     
     
         19 . The semiconductor device having epitaxial structures according to  claim 18 , wherein a concentration of the second semiconductor material in the undoped under layer is upwardly increased.