Dissolution of Germanium in Aqueous Hydrogen Peroxide Solution

The dissolution of germanium in aqueous hydrogen peroxide of from 3 to 90% H~O~ and pH from 1 to 9 has been studied at temperatures varying from 25 ~ to 90~ It has been found that dissolution rate remains constant in solutions of 3-30% by weight of H~O~ at low pH, then decreases with increasing H~O. _. content. The reaction is highly pH dependent above pH 4, and this pH dependence varies as a function of H~O. ~ concentration. This indicates that the rate-controlling step in low and high H. ~O~ concentration solutions at pH 5 and above may be different. Reaction mechanism to explain this situation is proposed. The use of hydrogen peroxide solution as a final clean-up etch for germanium is an established practice in the semiconductor industry. The use of dilute aqueous hydrogen peroxide solutions as an etch for germanium was first suggested by Camp (1). Miller (2) determined the dissolution rate of germanium at various temperatures in 3.4% by volume hydrogen peroxide solutions of pH 5. The present study has been carried out in order to obtain a better understanding of the mechanism of the dissolution reaction and to determine its dependence on the hydrogen peroxide concentration, temperature, and pH of solution. The effects of the resistivity, resistivity type, and crystalline perfection of the germanium on the dissolution rate also have been studied. Experimental The samples used were 1 x 1.5 x 0.010 in. single crystal n- and p-type germanium of 2 to 6 ohm-cm resistivity. 1 Samples were oriented with the major surfaces in the ~111~ plane within one degree and were prepared with an aluminum oxide lapped finish. Random area measurements on the slices showed less than 2% variation in apparent surface area. Resistivity measurements on individual samples showed variations less than 5%. Prior to etching, each sample was degreased in trichloroethylene, chloroform, and methanol, dried, cleaned in hydrofluoric acid, and rinsed in distilled, deionized water of better than 15 megohm resistivity. After drying, samples were weighed to the nearest hundredth of a milligram and thickness was measured with a dial micrometer to the nearest tenth of a mil. Samples were placed in small passivated stainless steel wire baskets and were etched individually in separate beakers containing 150 ml of solution, each maintained within 176 of temperature. No agitation of the etchant was used. The heat of reaction was low and quickly dissipated through the beaker so that the reaction was effectively carried out at constant temperature in all cases. At the end of each etch period, the germanium samples were 1 Specific experiments were carried out with germanium specimens of other resistivity ranges. quickly removed and quenched in a large volume of deionized water. After rinsing and drying, the samples were measured and weighed again. Samples were etched a number of times in the same solution in order to establish a constant etch rate. The assumption was made that the constant etch rate reached after the work damaged layer had been removed was close to the true etch rate. Since the decrease in thickness per etch period was below the sensitivity of the dial micrometer for the slow etching in very high concentration hydrogen peroxide, the majority of data for etch rate is presented in units of milligrams per square centimeter-hour, in order to be consistent from low to high hydrogen peroxide concentrations.