Magnetotransport investigations of the two-dimensional metallic state in silicon metal-oxid-semiconductor structures

For more than two decades it was the predominant view among the physical community that the every two-dimensional (2D) disordered electron system becomes insulating as the temperature approaches the absolute zero temperature (0 Kelvin or -273.15oC). Two-dimensional means that the movement of the charge carriers is confined in one direction by a potential so that the carriers can move freely only perpendicular to the confinement. The most famous physical realization of a 2D system is the silicon metal-oxide-semiconductor field effect transistor (Si-MOSFET). It is one of the basic elements of most electronic devices in our daily life. The working principle is very simple. Charges are attracted to the semiconductor-oxide interface by an electric field applied between the metallic gate and the semiconductor, so that a 2D conductive channel is formed. The charge density can be adjusted by the voltage from zero up to 1013 cm-2. In 1994 Kravchenko and coworkers made a very important discovery. They studied high mobility Si-MOSFETs and found that for densities below a certain critical value, nc, the resistivity increases as the temperature is decreased below 2 K, whereas for densities above $nc$ the resistivity decreases unexpectedly. The transition from insulating to metallic behavior, known as metal-insulator transition (MIT), was obviously a contradiction to the commonly accepted theories which predict insulating behavior for any density. The insulating behavior is a consequence of the wave properties of electrons which leads to interference in disordered media and thus to enhanced backscattering. In the subsequent years, experimental studies were performed on a variety of 2D systems, which qualitatively showed a similar behavior. All the investigated samples had one thing in common. The interaction energy between the carriers was considerable higher than their mean kinetic energy due to their movement in the 2D plane. Since the electron-electron interaction was only incompletely considered in the conventional theories, it seemed to be clear that interaction is the key to understand the anomaly. Several theories were put forward to explain the observations, including quantum phase transitions, formation of highly correlated electron liquids and new kinds of superconductivity. In this work, the temperature and the magnetic field dependence of the resistivity of Si-MOSFETs has been investigated down 50 mK. The specimens exhibit a pronounced MIT at low electron density and are similar to those used by Kravchenko and coworkers. In order to reveal the nature for the MIT, the threshold temperature Tq for the appearance of quantum interference was determined from the low-field magnetoresistivity. For temperatures below Tq, quantum interference has a considerable effect on the resistance, whereas above Tq, it can be ignored. It turns out the strong resistivity drop at medium densities occurs completely in the regime where quantum interference is not effective. On the other hand, interference effects have been observed down to the lowest temperatures at all densities studied. Thus the existence of a new kind of superconductivity in high mobility silicon inversion layers can be excluded as well. All indications, we have found, point to a classical phenomenon rather than to an interaction-induced phase transition. Consequently the measured resistivity was compared with different classical theories. One finds that the temperature dependence of the resistivity and its response to a magnetic field can be largely understood by assuming screened impurity scattering. Temperature and magnetic field changes the ability of the electron gas to screen impurities close to the 2D layer, so that scattering is more or less effective. Despite the success of the screening theory there are still a number of open questions which needs to be answered. (author)