Method for fabricating Group III nitride compound semiconductor substrates and semiconductor devices

A method for fabricating a Group-III nitride compound semiconductor substrate comprising epitaxial growth of a Group-III nitride compound semiconductor layer on a substrate base, and removal of the substrate base, said method comprising:

• forming an underlying layer on a substrate base, the underlying layer comprising at least one Group-III nitride compound semiconductor layer, and the uppermost layer of the underlying layer being a first Group-III nitride compound semiconductor layer;
• forming a first trench/mesa structure in which a portion of the first Group-III nitride compound semiconductor layer and a portion of the underlying layer are etched so as to form an island-like structure, thereby providing said first trench/mesa structure, such that the substrate base is exposed so as to provide the bottoms of trenches in said first trench/mesa structure;
• forming a first mask structure in which first masks are formed at the bottoms of the trenches in the first trench/mesa structure, such that the upper surfaces of the first masks are positioned below the top surface of the uppermost layer of the underlying layer;
• performing a first lateral epitaxial growth in which a second Group-III nitride compound semiconductor layer is epitaxially grown, vertically and laterally, with top surfaces of mesas and sidewalls of the trenches serving as nuclei for crystal growth, thereby filling spaces above the first masks and covering the mesas in said first trench/mesa structure;
• forming a second trench/mesa structure in which substantially the entire underlying layer that has not undergone etching in the formation of the first trench/mesa structure and a portion of the second Group III nitride compound semiconductor layer formed on the underlying layer are removed by etching, so as to provide said second trench/mesa structure including mesas formed on the remaining second Group-III nitride compound semiconductor layer and trenches such that the substrate base is exposed so as to provide the bottoms of the trenches of the second trench/mesa structure;
• forming a second mask structure in which second masks are formed at the bottoms of the trenches in the second trench/mesa structure, such that the upper surfaces of the second masks are positioned below the top surface of the second Group-III nitride compound semiconductor layer;
• performing a second lateral epitaxial growth in which a third Group-III nitride compound semiconductor layer is epitaxially grown, vertically and laterally, with top surfaces of the mesas and sidewalls of the trenches of the second trench/mesa structure serving as nuclei for crystal growth, thereby filling spaces above the second masks and covering the mesas of the second trench/mesa structure;
• removing the first and second masks by wet etching;
• and
• removing the substrate base, to thereby produce a substrate comprising the remaining second Group-III nitride compound semiconductor layer and the third Group-III nitride compound semiconductor layer.