Etching Ge with Mixtures of HF-H[sub 2]O[sub 2]-H[sub 2]O

Abstract

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...

Chemical Etching of Germanium in the System HF-H[sub 2]O[sub 2]-H[sub 2]O

“…In general, the rate numbers obtained in this work were comparable, to within ___5%, to the numbers obtained by Bloem and van Vessem (10). I I I I I I I I I I I 2 The H20 axis represents added water, not total water in the system.…”

“…For that reason, an oxidizing agent is always present in the solution 2 when either silicon (6,7) or germanium (8)(9)(10)(11) is etched. In most cases nitric acid is used as the oxidizing agent and hydrofluoric acid is added as the means of forming a soluble or gaseous reaction product (6,8,11).…”

Near-infrared emission from mesoporous crystalline germanium

“…13 Furthermore, the chemically dissolved samples exhibits progressive blue-shift, the peak positions are found at 1200 nm and 1095 nm (1.03, 1.14 eV) respectively for 30 min and 60 min etching in highly concentrated HF solution, which is known to etch Ge at a very low rate. 42 While no PL signal could be detected for the samples that were etched 90 min. Since only the size of the crystallites is shrinking with etching time and no composition / surface chemistry change is expected in this experiment, it is clear that this continuous blue-shift is yet another strong indication of a size-related emission which is analogous to the first demonstration of quantum-size related visible emission in porous Si by Canham in 1990.…”

“…To further reduce the crystallites size, the wafer is cleaved into separate pieces and dipped in the same electrolyte for extended chemical etching. Concentrated HF is used because it etches Ge chemically at an extremely slow rate (<0.05nm/min) 42 assuring a controllable reduction of crystallites dimensions.…”

The experimental setup of the Interaction in Crystals for Emission of RADiation collaboration at Mainzer Mikrotron: Design, commissioning, and tests

“…• Chemical etching has been used to cut the wafer obtaining rectangular samples; Au and Cr etchants have been used to remove the coating layer; 48 the metal residuals on the surface have been measured with the Rutherford backscattering spectrometry analysis. • The samples have been chemically treated by means of a calibrated low rate etching procedure, in order to reduce the thickness, 49 and mounted on custom mechanical holders (Figure 2). • The samples planarity and thickness homogeneity have been finally checked with HRXRD and X-ray absorption, 50 respectively.…”