Recent investigations have shown that cycloaddition reactions, widely used in organic chemistry to form ring compounds, can also be applied to link organic molecules to the (001) surfaces of crystalline silicon, germanium, and diamond. While these surfaces are comprised of Si=Si, Ge=Ge, and C=C structural units that resemble the C=C bonds of organic alkenes, the rates and mechanisms of the surface reactions show some distinct differences from those of their organic counterparts This article reviews recent studies of [2 + 2], [4 + 2] Diels-Alder, and other cycloaddition reactions of organic molecules with semiconductor surfaces and summarizes the current understanding of the reaction pathways.

We have obtained the first energy-resolved real-space images of the filled and empty surface states of the Si(l 1 l)-(7x7) surface, with 3-A lateral resolution. This ability to resolve spatially these surface states with a scanning tunneling microscope depends upon a new method to acquire and separate geometric and electronic information. Our results not only are in good agreement with previous spectroscopic studies but also directly reveal the atomic location and geometric origin of the Si (111) -(7 x 7) surface states.Surface states play an important role in the physical properties of solid surfaces and have been the subject of numerous theoretical and experimental studies. While these states have been extensively studied in reciprocal space, real-space observation and determination of the geometric origins of surface states have hitherto not been possible. We have atomically resolved and directly identified the physical origin and nature of the various surface states of the Si(lll)-(7x7) surface found in previous photoemission and inverse-photoemission studies. These include surface states due to dangling bonds on the twelve adatoms, several states localized on the atoms in the layer beneath the adatoms, a state due to Si-Si backbonds, and a state localized in the deep corner hole of the Si(lll)-(7x7) surface. In effect, we have mapped out the electronic states of the Si(lll)-(7x 7) surface with a lateral resolution of 3 A,The utilization of scanning tunneling microscopy (STM) to probe surface electronic structure in real space has been plagued by a number of experimental problems. 1 In one reported approach, 2 " 4 / vs V or dl/dV vs V curves are measured without scanning. While such curves contain electronic structure information, the lateral position of the tip is uncertain and real-space images of the surface states are not obtained. Alternatively, images of dljdV may be acquired at various bias voltages. 4 However, at each voltage the tip follows a different contour 5 so that the dl/dV images contain a mixture of geometric and electronic structure information, complicating interpretation of the observed features. 6 Ideally, one would like to know the complete I-Vcharacteristics, at constant sample-tip separation, at each point in a topographic image.Here we describe a new method, current-imagingtunneling spectroscopy (CITS), which overcomes the problems inherent in the aforementioned techniques and allows real-space imaging of surface electronic states. In this method the feedback control circuit used for constant-current STM is gated so that it is only active about 30% of the time. When the feedback loop is active, a constant voltage is applied to the sample; when inactive, the position of the tip is held stationary and the tunneling current is measured at various different bias voltages. By the repeating of this sequence at 2.2 kHz a constant sample-tip separation is maintained during all I-V measurements and the I-V characteristics of each point along the raster scan are determined simultaneously with t...

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