Epitaxial Growth of Germanium on Single Crystal Spinel

Films of germanium have been grown on single crystal spinel, MgAl 2 O 4 grown with excess Al 2 O 3 , via the pyrolysis of germane, GeH 4 . The films were single crystal when the growth temperature exceeded 500°C, but contained a high degree of twinning, strain, and grain boundaries. Film thicknesses have ranged from 0.2 to 49 µm. The electrical properties of films grown on (100) and (111) spinel have been measured as a function of growth temperature. The Hall mobility increased with increasing growth temperature, up to growth temperatures of about 750°C, where island growth began. Mobilities of the order of 300 cm 2 /vsec have been obtained in 7μ thick films. The acceptor concentration increased as the growth temperature decreased, and this increase in acceptor concentration has been attributed to increased defect density, the defects acting as acceptors. The growth rate of the films was found to increase with increasing growth temperature with an activation energy of 47.5 kcal/ mole. The films and substrate were deformed after growth due to the different coefficients of thermal expansion between germanium and spinel and this deformation has been used to estimate that the films were under a compressive stress of about 10 9 dynes/cm 2 . The deformation of Ge on spinel is not as great as Si on spinel or sapphire reflecting both the closer match in coefficients of thermal expansion and the decreased temperature change from growth temperature to room temperature.