AS CAPPING OF MBE-GROWN COMPOUND SEMICONDUCTORS - NOVEL OPPORTUNITIES TO INTERFACE SCIENCE AND DEVICE FABRICATION

In situ condensation of an amorphous cap of the high vapour pressure element (i.e., As, Sb) has been found to provide effective protection of molecular beam epitaxy grown compound semiconductor surfaces against ambient contamination. Most work reported so far relates to arsenic-capped AlGaAs. Detailed investigation with surface sensitive structural (RHEED, LEED) and chemical (XPS) probes confirms that the protective cap is conveniently removed by annealing in ultrahigh vacuum environments at a temperature in excess of similar to 350 degrees C. Clean AlxGa1-xAs(001) surfaces with different atomic reconstructions and corresponding (AI)Ga:As composition ratios are now routinely prepared by this technique, and thus offers an ideal testing ground for compound semiconductor surface and interface research. Reconstruction-dependent reactivity at metal/GaAs(001) interfaces is demonstrated, using surface sensitive synchrotron radiation photoelectron spectroscopy. Exploiting the protection offered by the As (Sb) cap for device fabrication purposes (e.g., in selective area epitaxy), demands a suitable method of pattern definition in the amorphous arsenic layer. The cap is shown to be chemically stable versus exposure to standard photolithographic processing chemicals, including photoresist, developer, and acetone (the photoresist solvent). However, the temperature required for thermal decapping is grossly inappropriate for photoresist curing. A novel technique of reactive decapping in a beam of hydrogen radicals (H*) is shown to be effective at room temperature. This innovation makes pattern definition in the As cap compatible with standard photolithography, and test structures with similar to 5 mu m linewidth is demonstrated. Scanning electron micrographs unveil the presence of arsenic cap residues along the photoresist mask edges. Moreover, trace amounts of surface gallium oxide and carbon impurities were found with core-level photoelectron spectroscopy. The technique thus needs further refinement before being useful in fabrication of compound semiconductor device structures.