Etch rate vs. composition of the etchant (HF-H~O~-H20) was studied, together with the surface structures after etching Ge single crystals. Conclusions concerning the orientation dependence and the kinetics of the etching process are given. The ratio of the etch rate on different faces depends on the value of CH~o/CHF in the etchant independent of the H~O~ concentration, whereas visible oxide films are formed for a CH..,oJCH~ ratio smaller than the stoiehiometric ratio given by the over-all reaction:Much work has already been done on the etching of Ge single crystals with mixtures of HF-H~O~ and H~O (1-9). This work has been extended by studying the etch rates on (111) oriented Ge crystals at room temperature as a function of etchant composition and by a study of the surface structures after etching.The Ge pellets used (dimensions 3 x 3 x 0.25 ram, 2-3 ohm-cm N-type) were cut along (111) planes to within 0.5 degree, the surfaces were polished with No. 500 carborundum abrasive powder. Etching was performed in a polyethylene vessel, rotating in a constant temperature bath (25~The vessel was inclined in order to be sure of adequate stirring. About 50 crystals at a time were etched in 250 cc of etchant.After etching the etchant was quenched with water and poured off After washing and drying the thickness of the pellets was measured to within 1;,. Etching times were used to give a total decrease in thickness of about 100m in each case. This procedure essentially follows that during fabrication, and the influence of damaged surface layers on the etch rate is small. The etch rate has been defined as that on a single surface in ~/min, thus half the decrease in thickness per unit time. Etch RatesThe results are plotted in a tri-axial diagram (Fig. 1), where the basic components in the three corners correspond to pure water, concentrated HF (50%), and H~O~ (30%), respectively. The concentrations are expressed volume percent. Each composition corresponds to a certain point in the diagram to which in turn the appropriate etch rate can be attached. In Fig. 1 contour lines of equal etch rates, at room temperature, on a single Ge surface are given.Maximum etch rates are found for about equal volume parts of HF and H~O.~; on dilution with water the etch rate decreases. As the I-IF contains 28.8 moles HF/1 and the H.~O~ used 9.8 moles/l, equal volume parts of both compose a nearly stoichiometric etchant according to the over-all reaction:Ge + 2H~O._, + 6HF ~ H~GeF~ + 4H~O[ 1 ]Figure 1 also shows that for low H~O~ or HF concentrations the etch rates vary proportional to this H20~ or HF content. The etch rates are strongly temperature dependent; a plot of log R over 1/T shows that for the stoichiometric etchants a variation in etch rate of about 6% per degree at room temperature is found (activation energy of 10.7 kcal/mole). It is quite possible that with less H~O~ or HF another activation energy is obtained, as other reaction steps may then become rate limiting. The experimental results at room temperature can be described by the...
In measuring t i e D.C. of a powder a grain-size effect must bz taken into account.For r,/fr ratio':; up to 3 special precautions will not he necessary. lor ratio's up to 6 the effect may be eliminated by using a large condenser (500 cm?) and a powder of fine grain-size (0.02 m m ) .Former measurements of several authors cannot be relied upon, thc grain-size effec': not having been taken into account. Measurements of the D.C. of NaCl and KCI powder at different fillingfractions under ideal conditions of powder-sizes and of NH,CI and KC1 floating in CCI,-C,,H, prove that the Biitfcher and Wiener-Bruggcrnan powder-formula'. give good results. In contrast with liquids the accurate D.C. measurements of solids meets with several difficulties. Each of the four methods, which may be used has certain disadvantages which makes the result of the measurement less accurate than would he desired: 1. Capxifly mcasurcnient of largc, flat-grohnd discs of crystal, ccpippcd with r?lcta/ elccfrodcs. From the dimensions of the crystal and the measured capacity the D.C. of the solid may be calculated directly with the aid of Kirchhofj's formula 1 ) c = capacity of a circu!ar condenser r = radius d = plate distance T h e disadvantages in this c x e m e : ( a ) Difficulties concerning the exact measurement of the physical dimensions of the crystal. ( b ) Uncertainty about the edge-correction terms in Kirchlioff's formula : I ) .
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