Effects of High Energy Ion Irradiation on Crystallization of Amorphous Germanium Films Deposited on Calcium Fluoride Sub
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the increase in defect density and nucleation and growth of randomly oriented grains.To obtain higher quality films, it is necessary to control and to reduce the defect density and the grains of
random orientation. In this study, a-Ge films are irradiated with 0.9 MeV Ge or Si ions at low IC100 nA/cm 2 and their structural changes are examined by Rutherford backscattering spectrometry (RBS) -channeling technique and thin film x-ray diffraction (XRD) measurement. In addition, we also examine the influence of deposition temperature (Td) of the Ge films on the crystallization induced by ion irradiation. EXPERIMENT Amorphous (a-) Ge films were prepared on air-cleaved CaF2 (I 11) substrates at a pressure of about 10-4 Pa by using an electron-beam-heating evaporation system. Deposition temperature (Id) was room temperature (RT) or 200 °C at which the film remained amorphous structure. Samples were cut into small pieces about 7 mm square and held in a sample holder in which the surface of the Ge films could be exposed to the irradiation beam within an area of 4 mm in diameter. The sample holder was attached on a 5-axis goniometer head, which allows parallel movement along the x-, y- and z-axes, tilt around the vertical axis (0), and tilt around horizontal axis parallel to the sample strface (ý). The irradiation was performed with 0.9 MeV Ge- and Si- ions in random (5' off) direction with a 1.7 MV tandem-type ion accelerator 171. The projected ranges calculated by the 191 Mat. Res. Soc. Symp. Proc. Vol. 396 0 1996 Materials Research Society
TRIM code were about 0.5 and 1.1 pm for Ge and Si ions, respectively. Ion current intensity (Ic) was 100 nA/cm 2. Ion dose was changed from 0.5 to 1.5 x 0151 ions/cm 2. Sample temperature during ion irradiation was measured by a pyrometer. The sample was heated by insulated heating wire for 1hour before irradiation and the temperature during irradiation was about 380 'C. Rutherford backscattering spectrometry (RBS) combined with channeling technique with 1.8 MeV He+ ions was used to determine the axis of the sample and to evaluate the magnitude of the crystallization of the Ge films as an in situ monitor. From the result of RBS measurement, areal density (or thickness) of Ge film was also obtained with the simulation program of RUMP code. In this experiment, the areal density of the Ge films was about 500 x 1015 atoms/cm2 (thickness c.a. 120 nm) inaverage. Thin film x-ray diffraction (XRD) measurement was performed by 20-scan operation system with incident x-rays at a small angle, which allowed us to check the presence of the randomly oriented grains without strong peak from single crystal phase. RESULTS AND DISCUSSION Figure 1 shows typical result of RBS aligned and random spectra of the sample after ion irradiation. The irradiation was performed with 0.9 MeV Ge ions at Ic= 100 nA/cm 2 and a dose of 1.5 x 1015 ions/cm 2 .The Ge film was prepared at Td = RT. Solid line indicates the result of simulation by RUMP code which gives us areal density of Ge. It is apparent that the aligned
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