Germanium etching

In conclusion Si etching is essentially controlled by the surface chemistry and diffusion of fluorine through the overlayer, as in the previous example, whereas germanium etching depends mainly on the fluorine flux with a weak effect of the surface chemistry. 

Fig. 1. Dislocation sites and sites of oxide nucieatlon on a 100 surface of germanium. Etched in CP4 and oxidized in poor vacuum.

Batterman (31) has attempted to explain the appearance of facets on hillocks in terms of only the measured orientation dependence of the etch rate. Using this, he concluded that the 322 planes are stable hillock facets on germanium etched with an HF + H2O2 + mixture. Irving (32) in a further analysis... 

Fig. 5. ill surfaces of germanium etched for 1 minute (see text> (a) Lapped surface, (b) Chemically polished surface. X200, before reduction for publication. 

H. SELLO (Fairchild Semiconductor) You state that p-type silicon and germanium etch at the same rate electro-lytically. In light of the different mechanisms, how do you account for this ... 

Videm, K., Pitting Corrosion of Aluminium in Contact with Stainless Steel , Proc. Conf. on Corrosion Reactor Mater., Salzburg, Austria, 1, 391 (1962) C.A., 60, 1412g Lyon, D. H., Salva, S. J. and Shaw, B. C., Etch Pits in Germanium Detection and Effects , J. Electrochem. Soc., 110, 184c (1963)... 

Turner DRJ (1961) Saturation currents at n-type silicon and germanium electrodes in chemical etching solutions. Electrochem Soc 108 561-563... 

Fig. 11. Capacitive transient spectra of defect states associated with dislocations in ultra-pure germanium (crystal 281 grown along [100] under 1 atmosphere of H2) The micrographs show the etch pits produced by dislocations on a (100) surface. DLTS peak b has an activation energy of Ev + 20 meV. The net-shallow acceptor concentration is 1010 cm-3.

Despite the fact that germanium is etched at the faster rate for a given set of plasma conditions, it does not become incorporated into the polymer matrix under the conditions of these experiments. This provides a convenient method to prepare films containing no cathode material but are essentially plasma polymerized polytetra-fluoroethylene. The small oxygen signal observed in the ESCA spectrum of this... 

Germanium surface passivation by chloride termination inhibits oxide formation and maintains a well-ordered surface. The chloride-terminated surface can also be used as a reactive precursor for wet organic functionalization. For example, Cullen et al. [105] first demonstrated the reaction of a chloride-terminated Ge(lll) surface with ethyl Grignard as a means of ethylation for use in surface stabilization. The chlorination was performed by a mixture of Cl2 and HC1 gas with N2 above atmospheric pressures [105]. Although this resulted in approximately a one-to-one ratio of adsorbed chlorine atoms with Ge surface atoms, the high pressures resulted in severe etching of the substrate [105]. 

Despite the range of hydrides present, hydride termination by HF etching stabilizes the surface against oxidation and maintains surface ordering for further wet chemistry. Hydride-terminated germanium shows no oxidation after exposure to ambient... 

Ma, Q., Moldovan, N., Mancini, D. C. and Rosenberg, R. A. Synchrotron-radiation-induced wet etching of germanium. Applied Physics Letters 81, 1741-1743 (2002). 

Ikeda, K., Imai, S. and Matsumura, M. Atomic layer etching of germanium. Applied Surface Science 112, 87-91 (1997). 

Fig. 35. Series capacitance data for the (111) faces of (a) near intrinsic (i-Ge) germanium, (b) n-Ge (0.0112 cm), and (c) p-Ge (0.112cm) in 0.1MNa2SO4 after etching in CP4. The electrodes were all polarised anodically at ca. lOOpAcm for a few minutes and the capacitance was measured immediately (within seconds) after polarising the electrode to the appropriate potential.

In the case of germanium, the formation of an oxyfluoride layer does not inhibit the etching in SF6—02 mixtures (only one slope), and for CF4—02 there is no dependency of the surface reactivity with the oxygen flux (null slope means Kd y>> Ka). Oxygen addition only provokes a modification of the fluorine concentration in the gas phase. Once more the energy brought by the ion bombardment explains these differences. 

The effect of gases on the oxide-covered germanium surface has been extensively studied over the past few years. The oxide-covered surface is usually produced by means of an aqueous etch. It is not stable in air, but changes rapidly during the first day and somewhat more slowly thereafter. Its structure and chemical composition are unknown and its thickness may vary from a few angstroms to as many as a hundred. 

This is illustrated in Fig. 1 the oxidation was carried out at a pressure of approximately 10" mm and at 900°C, at this temperature the oxide is volatile thus a pit is formed. Fig. 2 shows oxidation at a similar pressure on ill material although there are fewer oxidation pits in this picture, they are not found at dislocation sites. Oxidation in concentrated nitric acid had the advantage that the oxide layer left by the CP4 etch that was used to remove the damaged layer could be removed by HF. Many more oxide particles were formed on surfaces that did not have this oxide layer removed prior to oxidation. Thus we can say, at least for germanium, that the dislocations do not act as preferential sites for the nucleation of oxidation. 

Fig. 1 (a) Scratch in etched germanium surface produced by 600 mesh SiC. One of the abrasive particles is shown. 

The rocking-curve method has been applied to semiconductor materials by several workers. Weissmann(8) estimated 5ft as the depth of damage on germanium lapped with No. 305 abrasive (3200 mesh alundum, having nominal particle size of 5fi). On silicon Andrus and Bond (9) found l/2ft depth for a fine polish, 3ft for No. 305 lap, and 10ft for a diamond saw-cut the widths at half-maximum intensity before any etching were 13<... 

Fig. 4. Rate of etching as a function of time of etching and crystal orientation. Germanium surfaces had been finely ground with No. 305 abrasive (13).

Camp (11) determined depths of damage of 7, 5, and 3/a on (100), (111), and (110) faces of germanium lapped with No. 305 abrasive, and discussed the shapes of the curves for different faces in terras of the relative etching rates on different faces. 

Faust (1.3) used die etching rate method on both german-ium and silicon and covered a wide range of abrasive particle sizes. Some of his data are shown in Fig. 5 (14), He found that for a given abrasive treatment the depth of damage on silicon was about one fourth that on germanium. 

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