Passivation of semiconductor surfaces against chemical attack can be achieved by terminating the surface-dangling bonds with a monovalent atom such as hydrogen. Such passivation invariably leads to the removal of all surface states in the bandgap, and thus to the termination of non-metallic surfaces. Here we report the first observation of semiconductor surface metallization induced by atomic hydrogen. This result, established by using photo-electron and photo-absorption spectroscopies and scanning tunnelling techniques, is achieved on a Si-terminated cubic silicon carbide (SiC) surface. It results from competition between hydrogen termination of surface-dangling bonds and hydrogen-generated steric hindrance below the surface. Understanding the ingredient for hydrogen-stabilized metallization directly impacts the ability to eliminate electronic defects at semiconductor interfaces critical for microelectronics, provides a means to develop electrical contacts on high-bandgap chemically passive materials, particularly for interfacing with biological systems, and gives control of surfaces for lubrication, for example of nanomechanical devices.
The infrared absorption spectra of aqueous methanol, ethanol, 1- and 2-propanol, t-butanol, and n-butoxyethanol in the region of asymmetric and symmetric C–H stretching modes (3200–2800 cm−1) have been measured at 25 °C as a function of alcohol concentration in the whole cosolvent mole fraction region. Measurements of adiabatic compressibility were also performed on the same water–alcohol solutions at 25 °C. The two sets of experimental data are compared and discussed in terms of a possible mechanism of molecular aggregation in the various regions of alcohol concentration.
We have studied the evolutions of surface electronic structure ͑Fermi surfaces and valence bands͒ by electron filling into a two-dimensional free-electronlike surface state, during adsorptions of monovalent metal atoms ͑noble metal; Ag, and alkali metal; Na͒ on the Si͑111͒ ͱ 3 ϫ ͱ 3-Ag surface. The Fermi surfaces ͑Fermi rings͒ of a small electron pocket grow continuously with the adsorption. Eventually, when the ͱ 21ϫ ͱ 21 superstructure was formed by 0.1-0.2 monolayer adsorption of Ag or Na, the Fermi ring is found to be larger than the ͱ 21ϫ ͱ 21-surface Brillouin zone ͑SBZ͒, and to be folded by obeying the ͱ 21ϫ ͱ 21 periodicity. As a result, the Fermi surface is composed of a large hole pocket at the ⌫ point and small electron pockets at the K point in each reduced ͱ 21ϫ ͱ 21 SBZ, meaning that the behavior of surface-state carriers becomes hole-like. Despite a sharp chemical distinction between the adsorbates, a very similar surface electronic structure is found for both the Ag-induced and Na-induced ͱ 21ϫ ͱ 21 phases. Based on the Boltzmann equation, surface-state conductivites of these surfaces are obtained from the measured Fermi surfaces, reproducing successfully the results of previous surface transport measurements.
Surface metallization of SrTiO3(001) by hydrogen adsorption is experimentally confirmed for the first time by photoemission spectroscopy and surface conductivity measurements. The metallic state is assigned to a quantized state in the space-charge layer induced by electron doping from hydrogen atoms. The measured two-dimensional (2D) conductivity is well above the 2D Ioffe-Regel limit indicating that the system is in a metallic conduction regime. The mean free path of the surface electron is estimated to be several nanometers at room temperature.
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