Grain growth and grain size distributions in thin germanium films

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Abstract
We have observed and characterized normal and secondary grain growth in thin films of germanium on silicon dioxide. Films were deposited on thermally oxidized silicon wafers, encapsulated with 1000‐Å‐thick sputtered SiO2 films and annealed in evacuated ampoules at 900 and 915 °C. After 5 min, the films had developed a columnar grain structure as a result of normal grain growth. The grain size distributions were lognormal with mean grain diameters of about 2.5 times the film thickness. The standard deviation of the normal grain size distribution did not change significantly with annealing time and temperature. Secondary grain growth, which can lead to grains that are much larger than the film thickness, occurred in films that were annealed for longer periods of time. The normal grain size distribution remained stationary, i.e., the peak height and width did not increase with annealing time. Secondary grains were few in number compared to normal grains, and were manifest as a small tail on the normal grain size distribution. The rate of secondary grain growth was constant and largest in the thinnest films.