Germanium crystals, manufacture

Infrared optics is a fast growing area in which CVD plays a maj or role, particularly in the manufacture of optical IR windows. 1 The earths atmosphere absorbs much of the infrared radiation but possesses three important bandpasses (wavelengths where the transmission is high) at 1-3 im, 3-5 im and 8-17 pm. As shown in Table 16.2, only three materials can transmit in all these three bandpasses single crystal diamond, germanium, and zinc selenide. 

Once germanium is recovered and formed into blocks, it is further refined by the manufacturer of semiconductors. It is melted, and the small amounts of impurities such as arsenic, gallium, or antimony, are added. They act as either electron donors or acceptors that are infused (doped) into the mix. Then small amounts of the molten material are removed and used to grow crystals of germanium that are formed into semiconducting transistors on a germanium chip. The device can now carry variable amounts of electricity because it can act as both an insulator and a conductor of electrons, which is the basis of modern computers. 

For use in infrared optics, zone-refined germanium is recast or grown into forms suitable for lens and window manufacture. Polycrystalline castings of up to 115 kg and single crystals of up to 90 kg are routinely made. After the germanium is annealed, it is cut and ground into lens- or window-blanks, which are then polished, coated, and assembled into an ir system (see Infrared and raman spectroscopy, infrared technology). 

Atoms of boron and arsenic are inserted into germanium s crystal structure in order to produce a semiconducting material that can be used to manufacture computer chips. 

Yet another physical phenomenon is used in solid-state detectors, which are manufactured from high quality silicon or germanium single crystals doped with lithium and commonly known as Si(Li) or Ge(Li) solid-state detectors. The interaction of the x-ray photon with the crystal (detector) produces electron-hole pairs in quantities proportional to the energy of the photon divided by the energy needed to generate a single pair. The latter is... 

The early solid-state devices were not as sophisticated as devices produced today. A single transistor consisted of a relatively large crystal of germanium or silicon 0.25 inch square and 0.125 inch thick to which conductors were attached. The manufacturing processes were relatively insensitive to outside impurities. 

A diode is a nonlinear device that has greater conductance in one direction than in the other. Useful diodes are manufactured by forming adjacent //-type and />-type regions within a single germanium or silicon crystal the interface between these regions is termed a pn junction. 

Solid-state diffusion also plays an important role in the manufacture of semiconductors. To produce the junctions needed in these devices, a dopant such as boron is deposited on the surface of the semiconductor crystal, e.g., silicon or germanium, and is subsequently made to diffuse into the interior. This process, termed drive-in diffusion, is again carried out at elevated temperatures. An analysis of it and some relevant calculations appear in Chapter 4, Practice Problem 4.4. The diffusivity required in these calculations is derived in the short illustration given below. 

On metal electrodes, the transfer coefficients typically approach 0.5. Generally, the transfer coefficients for redox reactions on moderately doped diamond electrodes are smaller than 0.5 their sum a +p, less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, on p-type semiconductors, reactions proceeding via the valence band have the transfer coefficients a = 0, P = 1, and thus, a +p = 1 [7]. Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials manufactured by use of advanced technologies ( like germanium, silicon, gallium arsenide, etc.). The departure from the ideal semiconductor behavior is likely to be caused by the fact that the interfacial potential drop appears essentially localized, even in part, in the Helmholtz layer, due, e.g., to a high density of surface states, or the surface states directly participate in the electrochemical reactions. As a result, the transfer coefficients a and p have intermediate values, between those characteristic of semiconductors (O or 1) and metals (-0.5). Semiconductor diamond falls in with this peculiarity. However, for heavily doped electrodes, the redox reactions often proceed as reversible, and the transfer coefficients approach 0.5 ( metaMike behavior). 

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