Semiconductor energy resolution

In SAM the electron beam can be focussed to provide a spatial resolution of < 12 nm, and areas as small as a few micrometers square can be scanned, providing compositional information on heterogeneous samples. For example, the energy resolution is sufficient to distinguish the spectrum of elemental silicon from that of silicon in the form of its oxide, so that a contaminated area on a semiconductor device could be identified by overlaying the Auger maps of the two forms of silicon obtained from such a specimen. 

For metal/semiconductor interfaces the limitations coming from 1) are not very severe since the cross sections for photoemission and the lifetime broadening of such deep state photopeaks along with the poor energy resolution of the photon source... 

For X-ray detection, Charge-Coupled Devices (CCDs) are used. Being pixel detectors, they have a built-in two-dimensional position resolution and an energy resolution even better than conventional semiconductor detectors. They allow an efficient background reduction by analyzing the hit pattern and simultaneously... 

The requirements of high-energy resolution are well met by Si and Ge. The photoelectric effect increases with Z to Z, where Z is the atomic number of the substance, and the linear absorption coefficient for lOOkeV y rays in Ge is higher than in Si by a factor of about 40. Therefore, Ge crystals are better suited for measuring y radiation. Semiconductors with still higher atomic numbers, such as CdTe and Hgl2, have been investigated with respect to their suitability as detector materials, but they are not commonly used. 

Developed in the 1960s, semiconductors are the newest form of counter. They produce pulses proportional to the absorbed x-ray energy with better energy resolution than any other counter this characteristic has made them of great importance in spectroscopy (Chap. 15). Although they have had little application in diffraction, it is convenient to describe them here along with the other counters. 

Reflectance-based optical characterization techniques offer the advantages of high energy resolution and sensitivity to both macrostructural and microstructural effects while nondestructively providing real-time information with the sample in any transparent ambient. Experimental and analytical methods are discussed, and examples are given to illustrate representative applications to problems of current interest in semiconductor technology. 

Figure 3 demonstrates the electron spectrometer part of a depth-resolved conversion electron MOssbauer spectrometer specially designed for such measurements in our laboratory (10, 11). The electron spectrometer is of the cylindrical mirror type back-scattered K conversion electrons from resonantly excited Fe nuclei are resolved by the electrostatic field between the inner and outer cylinders (cylindrical mirror analyzer) and then detected by a ceramic semiconductor detector (ceratron). The electron energy spectra taken with this spectrometer indicate that peaks of 7.3-keV K conversion electrons, 6.3-keV KLM Auger electrons, 5.6-keV KLL Auger electrons, etc., can be resolved well, with energy resolution better than 4%. 

The most important advantage of the semiconductor detectors, compared to other types of radiation counters, is their superior energy resolution the ability to resolve the energy of particles out of a polyenergetic energy spectrum (energy resolution and its importance are discussed in Chaps. 9, 12-14). Other advantages are... 

To discuss the effect of the statistical fluctuations on energy resolution, consider a monoenergetic source of charged partieles being detected by a silicon semiconductor detector. (The discussion would apply to a gas-filled counter as well.) The average energy w needed to produce one electron-hole pair in silicon is... 

It can be seen from Eq. 9.7 that the resolution is better for the detector with the smaller average energy needed for the creation of a charge carrier pair (and smaller Fano factor). Thus, the energy resolution of a semiconductor detector (w 3 eV, F < 0.1) should be expected to be much better than the resolution of a gas-filled counter (w = 30 eV, F 0.2), and indeed it is (see Chaps. 12 and 13). 

Assume that the energy resolution of a scintillation counter is 9 percent and that of a semiconductor detector is 1 percent at energies around 900 keV. If a source emits gammas at 0.870 MeV and 0.980 MeV, can these peaks be resolved with a scintillator or a semiconductor detector ... 

For a good measurement, the amplifier should satisfy many requirements. Not all types of measurements, however, require the same level of performance. For example, if one measures only the number of particles and not their energy, the precision and stability of the amplification process can be relatively poor. It is in spectroscopy measurements, particularly measurements using semiconductor detectors, that the requirements for precision and stability are extremely stringent. Since the energy resolution of Ge detectors is of the order of 0.1 percent, the dispersion of the pulses due to the amplification process should be much less, about 0.01 percent. 

For Ge detectors other than the well-type, the efficiency is low, relative to Na(Tl) scintillation counters. This statement holds true for Si(Li) detectors as well (see Sec. 12.9). Lower efficiency, however, is more than compensated for by the better energy resolution of the semiconductor detector. Figure 12.32 illustrates the outstanding resolution characteristics of a semiconductor detector by showing the same spectrum obtained with a Nal(Tl) and a Ge(Li) detector. Notice the tremendous difference in the FWHM. The Ge(Li) gives a FWHM =... 

The width F is indicated as electronic noise in Fig. 12.41. Of the three types of X-ray detectors mentioned—scintillation, proportional, and semiconductor counters—the Si(Li) detector has the best energy resolution for X-rays. This fact is demonstrated in Fig. 12.42, which shows the same energy peak obtained with the three different detectors. Notice that only the Si(Li) detector can resolve and lines, an ability absolutely necessary for the study of fluorescent X-rays for most elements above oxygen. The manganese fluorescence spectrum obtained with a Si(Li) detector is shown in Fig. 12.43... 

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