3.6.1. Semiconductor Detector

The semiconductor detectors used in EDS analysis are proportional detectors like the gas-flow detectors used in WDS analysis. However, in contrast, it has good energy discrimination by itself and can be used without preselecting energies of interest using an analyzing crystal. Semiconductors detectors are single crystals of either Si or Ge. If the crystal structure were perfect, there would be no local abundances and shortages of electrons; however, there are imperfections (lattice defects, impurities, etc.) that result in electron-deficient areas (termed "holes") and extra free electrons within the crystal lattice. These holes and free electrons act as charge carriers when an electrical field is applied across the crystal. Thus, a pure crystal, with fewer holes and free electrons, allows less current to pass than an impure crystal.

Microprobes use Si(Li) semiconductor detectors, which consist of a 2 to 5 mm thick Si crystal, with gold contacts at its ends. A bias is applied across the crystal and a current flows (Figure 3.6.1a). The Si crystal consists of a "Li-drifted" intrinsic region facing the specimen and an adjacent Li-free region. The front contact, Li-drifted intrinsic region, and Li-free region constitute a p-i-n junction. The crystal is maintained at low temperatures to prevent diffusion of Li from the intrinsic region to the Li-free region. In general, the diffusion of Li is only a problem when there is a voltage across the detector.

Figure 3.6.1a. Cross section of a typical lithium-drifted silicon detector. X-rays create electron-hole pairs in the intrinsic region of the semiconductor; these charge carriers then migrate to the electrodes under the influence of an applied bias voltage (after Kevex Corporation 1983).

Immediately beneath the gold surface facing the sample is a "dead-zone" where the Li drifting is inadequate to deal with impurities. In this zone there are a great excess of holes (Figure 3.6.1b). This dead-zone traps charges produced by X-ray interaction in the crystal and produces a low-energy "tail" on the side of the peak.

Figure 3.6.1b. The X-ray detection process in the Si(Li) detector. Incident X-rays may cause ionization in the Si of the detector. The resulting characteristic X-rays may escape or be absorbed within the detector. The incident X-ray loses energy equivalent to Ec for Si. The energy of incident X-ray can also be absorbed by production of Auger electrons and by electron-hole pairs. (after Goldstein et al. 1981).

The covalent bonding electrons of the Si (or Ge) atoms occupy an energy level termed the valence band (VB). When an X-ray hits a crystal, it is absorbed by an atom and produces a high-energy photoelectron. The photoelectron knocks a VB electron into what is termed the conduction band (CB) leaving an "intrinsic" hole. This interaction uses up some of the energy of the photoelectron, and the slightly less energetic photoelectron continues to produce more CB electrons and holes until its energy is dissipated (Figure EDS-2). On average for a Si crystal, 3.8 to 3.9 eV are dissipated per hole created. Thus, X-rays with energy >1 keV make many holes.

The escape peaks in a Si(Li) detector are 1.84 keV lower in energy (E_c for Si-Ka) than the incident X-rays . Escape peaks will be present for all parent peaks above 1.84 keV (the Si absorption edge), and their magnitudes are usually a few percent of the parent peak.

A bias placed across the crystal causes the electron-hole pairs to migrate and increases the conductivity of the crystal. The more holes created, the greater the increase in conductivity and the associated drop in resistance. The total charge conducted is directly proportional to the energy of the absorbed X-ray. Unlike a gas-flow detector there is no internal gain.

Although the number of electron-hole pairs created is a function of the energy of the X-ray, it is a statistical process and there is a normal distribution around the actual X-ray energy. Detection is a statistical process and resolution is treated as for gas-flow detectors. Fano factor for Si(Li) semiconductor detector ranges from 0.1 to 0.13. Resolution depends not only on the detector but also the electronics. The standard industry EDS resolution test is on Mn-Ka radiation (5.9 keV) at 1000 cps with an 8 msec time constant. Resolutions about 150 eV in EDS analysis. WDS systems can handle a much higher count rates without loss of resolution than EDS systems. For WDS the limit is about 50000 cps, whereas EDS functions best at <2000 cps.>

Baseline conductivity due to thermal excitation of electrons from VB to CB in an imperfect crystal produces a leakage current and increased noise and backgrounds. One way to reduce the effects of the leakage current is to sheath the detector in a liquid-N₂-filled cryostat producing a temperature of about 77°K (-209°C). In addition, as noted above, Li drifting is used to compensate for impurities. Li is an electron donor and its presence makes the Si crystal a better semiconductor and swamps out the effects of impurities. Li-drifted crystals are called "silly" for Si(Li) and "jelly" for Ge(Li).

The preamplifier is located as close to the detector as possible to avoid amplification of electronic noise rather than signal. It incorporates a field-effect transistor (FET), which is used to reset the pulse amplification circuitry. The detector crystal and preamplifier are housed in a liquid N₂-cooled jacket separated from the electron column by a thin (8 mm) Be-foil window that is transparent to most X-rays of interest. The voltage pulses produced by the detector are fed into a linear preamplifier and then a multichannel analyzer (MCA).