Development of a Photoemission Microscope: Maximum and its Application to Semiconductor Interfaces.

Photoemission spectroscopy with synchrotron radiation is a well-established technique for analyzing the electronic and chemical properties of solids and surfaces. However, photoemission suffers from a major limitation in its application to heterogeneous systems, due to large probe size (100 mu m times 100 mu m). The development of a photoemission microscope with both high spatial and energy resolution that can overcome this limitation is thus of great interest, and it can provide unique and complementary information to other microscopy techniques. The project MAXIMUM is a scanning photoemission microscope located at the U41 undulator at the Synchrotron Radiation Center at the University of Wisconsin. The instrument is based on a multilayer -coated Schwarzschild objective, operating at 95eV, which forms a small probe on the sample. A cylindrical mirror analyzer is used to detect the photoelectrons emitted from the sample. The microscope has demonstrated spatial resolution better than 0.1mu m in transmission, and electron energy resolution of 250meV has also been demonstrated in photoemission. The typical operating condition of the microscope is carried out with sub-micron spatial resolution and 350meV energy resolution, due to the relatively large emittance of the undulator source. The design and the implementation of the MAXIMUM project are presented. The microscope is utilized to study the formation of the Ge/GaAs heterojunctions on cleaved GaAs(110). The results showed that, initial cleavage-induced defects on the cleaved GaAs substrate have a large effect on the value of the valence band off-set. These lateral inhomogeneities are undetectable with conventional photoemission, and it demonstrates the need for spatially resolved photoemission. The instrument was also utilized to study thick "buried" interfaces in cross-section. A GaAs homojunction was imaged by photoemission for the first time with 0.5mu m resolution. These first results from MAXIMUM demonstrate that the combination of microscopy and photoemission spectroscopy can greatly expand our knowledge of the surfaces and interfaces of semiconductors.