Atom probe semiconductors

Compositional variations across interfaces between metals and various semiconductors have also been analyzed with the atom-probe. 

FIGURE 40.27 Mass spectrum from a P-doped Ge nanowire as obtained by atom probe. Reprinted from Perea, D.E., Wijaya, E., Lensch-Falk, IL., Hemesath, E.R., Lauhon, L.I (2008) Tomographic analysis of dilute impurities in semiconductor nanostructures./owmal of Solid State Chemistry, 181(1), 1642-1649. Copyright (2008), with permission from Elsevier. 

Gorman, B.P., Norman, A.G., Yan, Y. (2007) Atom probe analysis of III-V and Si-based semiconductor photovoltaic structures. Microscopy and Microanalysis, 13, 493-502. 

Hashizume, T, Hasegawa, Y, Kobayashi, A., Sakurai, T. (1986) Atom-probe investigation of III-V semiconductors comparison of voltage-pulse and laser-pulse modes. Review of Scientific Instruments, 57,1378-1380. 

Heteroatoms adsorbed to the tip can also play a large role in determining the observed spectra. Many studies have shown that the transfer of an adatom or molecule to the STM tip can affect the observed topography, e.g. yielding atomic resolution on an electronically flat close-packed metal substrate. The spectroscopic effects can be even larger. Special care must be taken when probing semiconductor surfaces. Tip-sample contact resulting in some... 

AFM measures the spatial distribution of the forces between an ultrafme tip and the sample. This distribution of these forces is also highly correlated with the atomic structure. STM is able to image many semiconductor and metal surfaces with atomic resolution. AFM is necessary for insulating materials, however, as electron conduction is required for STM in order to achieve tiumelling. Note that there are many modes of operation for these instruments, and many variations in use. In addition, there are other types of scaiming probe microscopies under development. 

There are many other experiments in which surface atoms have been purposely moved, removed or chemically modified with a scanning probe tip. For example, atoms on a surface have been induced to move via interaction with the large electric field associated with an STM tip [78]. A scaiming force microscope has been used to create three-dimensional nanostructures by pushing adsorbed particles with the tip [79]. In addition, the electrons that are tunnelling from an STM tip to the sample can be used as sources of electrons for stimulated desorption [80]. The tuimelling electrons have also been used to promote dissociation of adsorbed O2 molecules on metal or semiconductor surfaces [81, 82]. 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

STM found one of its earliest applications as a tool for probing the atomic-level structure of semiconductors. In 1983, the 7x7 reconstructed surface of Si(l 11) was observed for the first time [17] in real space all previous observations had been carried out using diffraction methods, the 7x7 structure having, in fact, only been hypothesized. By capitalizing on the spectroscopic capabilities of the technique it was also proven [18] that STM could be used to probe the electronic structure of this surface (figure B1.19.3). 

Various Ru-oxides, YBa2Cu307, c (I), Ba Ru2/3Gdi/303 (II) as well as Ru-doped a-Fe203 (III), to probe the local chemical structure around the Ru atoms. Compound (I) has interesting properties with x < 0.2 it is a superconductor and with x 1 a semiconductor. Ru oxidation state and coordination are discussed on the basis of measured isomer shifts and quadrupole splittings Ru(IV) ions exclusively occupy Cu-1 sites which form one-dimensional chains... 

Abstract Silver clusters, composed of only a few silver atoms, have remarkable optical properties based on electronic transitions between quantized energy levels. They have large absorption coefficients and fluorescence quantum yields, in common with conventional fluorescent markers. But importantly, silver clusters have an attractive set of features, including subnanometer size, nontoxicity and photostability, which makes them competitive as fluorescent markers compared with organic dye molecules and semiconductor quantum dots. In this chapter, we review the synthesis and properties of fluorescent silver clusters, and their application as bio-labels and molecular sensors. Silver clusters may have a bright future as luminescent probes for labeling and sensing applications. 

An important aspect of semiconductor films in general with regard to electronic properties is the effect of intrabandgap states, and particularly surface states, on these properties. Surface states are electronic states in the forbidden gap that exist because the perfect periodicity of the semiconductor crystal, on which band theory is based, is broken at the surface. Change of chemistry due to bonding of various adsorbates at the surface is often an important factor in this respect. For CD semiconductor films, which are usually nanocrystalline, the surface-to-volume ratio may be very high (several tens of percent of all the atoms may be situated at the surface for 5 mn crystals), and the effects of such surface states are expected to be particularly high. Some aspects of surface states probed by photoluminescence studies are discussed in the previous section. 

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