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

In situ condensation of an amorphous cap of the high vapour pressure e lement (i.e., As, Sb) has been found to provide effective protection o f molecular beam epitaxy grown compound semiconductor surfaces against ambient contamination. Most work reported so far relates to arsenic-c apped 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 environme nts at a temperature in excess of similar to 350 degrees C. Clean AlxG a1-xAs(001) surfaces with different atomic reconstructions and corresp onding (AI)Ga:As composition ratios are now routinely prepared by this technique, and thus offers an ideal testing ground for compound semic onductor surface and interface research. Reconstruction-dependent reac tivity at metal/GaAs(001) interfaces is demonstrated, using surface se nsitive synchrotron radiation photoelectron spectroscopy. Exploiting t he protection offered by the As (Sb) cap for device fabrication purpos es (e.g., in selective area epitaxy), demands a suitable method of pat tern definition in the amorphous arsenic layer. The cap is shown to be chemically stable versus exposure to standard photolithographic proce ssing chemicals, including photoresist, developer, and acetone (the ph otoresist solvent). However, the temperature required for thermal deca pping is grossly inappropriate for photoresist curing. A novel techniq ue of reactive decapping in a beam of hydrogen radicals (H) is shown to be effective at room temperature. This innovation makes pattern def inition in the As cap compatible with standard photolithography, and t est structures with similar to 5 mu m linewidth is demonstrated. Scann ing electron micrographs unveil the presence of arsenic cap residues a long the photoresist mask edges. Moreover, trace amounts of surface ga llium oxide and carbon impurities were found with core-level photoelec tron spectroscopy. The technique thus needs further refinement before being useful in fabrication of compound semiconductor device structure s.