Investigation of electron-beam-induced phase transitions in amorphous aluminum trifluoride thin films using transmission electron microscopy

In this study we employ an electron probe of a current density similar to 10(4) Cm-2 from a conventional transmission electron microscope to examine in situ the electron-beam-induced crystallization behavior of thermally evaporated amorphous aluminum trifluoride (a-AlF3) thin films. Under the electron beam, the self-developing a-AlF3 material undergoes an extremely complicated crystallization process, subsequently forming crystalline Al, AlF3, and Al2O3 substances. The crystallization process of a-AlF3 relies heavily on the amount of H2O absorbed into the films, thereby reducing the dose required to crystallize a-AlF3, inhibiting radiolytic dissociation of AlF3, into aluminum and fluorine, and further activating a chemical reaction between AlF3 and the absorbed H2O, forming oriented Al2O3 crystallites. Dry a-AlF3 thin film material, evaporated from anhydrous AlF3 powder under a base pressure of 10(-4) Pa, follows a clear sequence sequentially transforming itself into aluminum, crystalline AlF3 (c-AlF3) and textured Al2O3 at a threshold dose of approximately 1 x 10(5), 1 x 10(6), and 1 x 10(7) Cm-2, respectively. However, a-AlF3 thin film material, deposited from hydrous AlF3. 3H(2)O powder under a relatively wet vacuum ambient of a base pressure of 10(-2) Pa, has a reduced threshold dose of about 2 x 10(5) Cm-2 to transform itself simultaneously into c-AlF3 and aluminum. Monitoring the oxygen 534 eV and fluorine 685 eV energy-loss spectra during the irradiation of AlF3 films indicates that: (a) c-AlF3 is far more resistant to radiolysis than a-AlF3 and (b) H2O is gradually absorbed into the films, ultimately initiating the transformation of AlF3 into Al2O3. Finally, the statistic variation toward less sensitivity of developing a-AlF3 resists found in previous investigations is attributed to the higher ability of the beam-crystallized AlF3 against radiolysis. (C) 1999 American Vacuum Society. [S0734-2101(99)02402-1].