Surface charge spectroscopy and its applications

Surface Charge Spectroscopy (SCS) is a surface sensitive technique for measuring potential distributions across an ultrathin (< 100 Angstrom) insulator/semiconductor structure. Although the applicability of the technique is rather narrowly confined to such a specific sample structure, SCS bears considerable industrial significance because insulator/semiconductor structures are the most commonly used functional elements in microelectronics, and because gate insulators used in the industry are indeed getting to a nanometer thickness scale. The basic SCS concept relies on the measurements of the potential energies of electronic states using the conventional photoemission spectroscopic method, energy data which bear the information of the electrical potential gradient across the probed surface region. Surface sensitivity of SCS is thus contained in the framework of photoemission spectroscopy. Unlike conventional photoemission studies, SCS always measures the potential gradients corresponding to a specific surface potential. In fact, its analytical power is only shown when the surface potential of a sample can be controlled, and coincidentally an insulator/semiconductor device structure in microelectronics is only functional when the potential gradient extending into the semiconductor region can be changed effectively by the surface potential of the insulator. When SCS data are examined beyond the simple space-charge model commonly employed by electrical characterisation techniques like capacitance-voltage measurements, they can give information other than the relationship between the insulator surface potential and semiconductor surface potential (and thus interface state distributions across the bandgap of the semiconductor). Rather, they also provide unique information on the depth distributions of various types of fixed charges in the sample structure at an atomic level and on the insulator breakdown mechanisms.