Ultra-Microscopic Analysis of Metal-Compound Semiconductor Interfaces by the Atom-Probe Mass Analyzer

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The voltage-pulse atom-probe(A-P) analysis of compound semiconductors tends to give an erroneous composition, e.g. more As ions than Ga ions are detected from GaAs. The newly constructed combined A-P of straight, deflected and imaging types made it possible to clarify the cause of the erroneous compositions as the preferential field evaporation of Ga immediately after the evaporation of nearby As atoms. Then it was successfully demonstrated that the A-P can give stoichiometric compositions by adjusting the deflector and pulse voltages.The unique features of the combined A-P are high ion detectability by a chevron electron multiplier with a wide effective incoming area and by a signal height discriminating time digitizer, high-speed data process by a 32-bit computer, and monitoring of an imaging A-P with a storage oscilloscope.Another anticipated approach to analyzing compound semiconductors is to promote field evaporation by irradiating a specimen with a pulsed-laser beam, 0.8 ns wide and a peak power of 200 KW. The present study pointed out that even the A-P analysis with a pulsedlaser fails to give a right composition unless the laser power and tip voltage are not properly adjusted. Utilizing the result of the present study, Al-GaAs interfaces were analyzed by the pulsed-laser A-P, comparing the result obtained by the voltage-pulse A-P. Unexpectedly, a Ga layer was found at the interface of an AlAs layer and the substrate GaAs. At present, the formation mechanism of the Ga layer is under investigation.