The transition from the crystalline to the amorphous state has been studied in germanium thin films by observing the electroreflectance spectra related to critical points at r (E Qi E 0 +A 0 ), along the A direction (2? 1 ,.E 1 + A 1 ), and at areas with A and 2 symmetry (E 2 ). A small shift of E 0 to lower energy with increasing disorder in the crystalline state, and weak structure in the amorphous state, have been observed. The spectral positions of E l9 E 1 +A 1 and E 2 do not change with disorder in the crystalline state but these responses are completely absent in the amorphous state.Increasing attention has recently been paid to the correlation between the electronic structure of a solid and the degree of long-range order in its lattice. 1 ' 2 Many experimental results suggest that the amorphous state retains a residual energy-band structure. 3 " 5 However, little is known about its detailed features, or how it evolves from the band structure of the crystalline state as disorder is gradually introduced.We report in this Letter the electroreflectance spectra of a series of germanium films deposited onto substrates of various temperatures. In such a set of samples, the long-range order is systematically reduced towards cooler substrates, and a transition from the poly crystalline to the amorphous state is passed. Static reflectance spectra, which for each photon energy monitor the sum of all possible transitions in the entire Brillouin zone, drastically lose the sharpness of their spectr v al features as the completely disordered state is approached. 3 * 5 In electroreflectance, only transitions near localized, structuresensitive discontinuities of the electronic densityof-states function respond to the electric-field perturbation. 6 In contrast to "integral" properties, electroreflectance is therefore expected to monitor more sensitively the gradual weakening of order-dependent features in the band structure as the long-range order is reduced.In a set of evaporated germanium films, the transition from "polycrystalline" to "amorphous" occurs at a substrate temperature T s t that depends upon evaporation rate, vacuum conditions, and substrate material. Accordingly, a wide range of values T / extending from 100°C to slightly below 400°C has been reported. In this study we closely controlled the evaporation conditions, so that mainly the substrate temperature T s , varied in steps of 25°C over the range 100 ^T s ^ 500°C, determined the crystalline order of the film. Controlled by an acoustic thickness monitor (Sloan), all samples of the set are 2400 ± 200 A thick, prepared at a growth rate of 7-10 A/sec in a vacuum of (4-7)xl0~7 Torr. Three different types of substrates were coated for each temperature T s . Two consisted of single-crystal germanium, polished to a final abrasive diameter of 1 JLL, one of which subsequently was coated with an opaque layer of aluminum. The film on the third substrate, a quartz flat, served for measurements of the electron diffraction pattern and of the space-charge capacitance. T...
