The structural properties (radial distribution function, small angle scattering, density), optical properties (complex dielectric function versus energy), and other characterizing parameters (scanning electron microscope examination, chemical analysis, recrystallization behaviour, transport phenomena), have been determined for a series of amorphous Ge films sputtered at different substrate temperatures. Various of these properties (e.g. refractive index, density, room temperature conductivity) show only slowly varying values for low and high substrate temperatures, with a rapid changeover in a critical temperature range. A model has been developed which relates this behaviour to the probability that an adatom configuration be able to change to a configuration of lower local free energy in the time for the deposition of a monolayer, and it is suggested that such behaviour may be a general feature of disordered films prepared by any deposition method. The structural and optical properties at a give...

Amorphous germanium I. A model for the structural and optical properties

Several investigators have analyzed optical data on amorphous Ge films prepared by evaporation on room-temperature substrates to derive a sharp absorption edge near 0.5 eV. A re-examination of their work shows that the sharp edges, found hitherto, may be artifacts resulting from either experimental or calculational errors and that the preponderance of the current evidence favors a gradual absorption edge.



Von einigen Autoren wurden die optischen Daten von amorphen Ge-Schichten, die durch Aufdampfen bei Zimmertemperatur hergestellt wurden, analysiert und daraus eine scharfe Absorptionskante bei 0,5 eV abgeleitet. Eine wiederholte Untersuchung ihrer Arbeiten zeigt, das die scharfen Kanten, die bisher gefunden wurden, Fehleffekte sein konnen, die entweder von experimentellen oder rechnerischen Fehlern herruhren und das die Mehrzahl der gegen wartigen Hinweise fur eine allmahlich ansteigende Absorptionskante spricht.

Comments on the evidence for sharp and gradual optical absorption edges in amorphous germanium

The optical properties of thick sputtered films (∼30μ) of amorphous Ge, grown with different substrate temperatures (0ˇ-T sˇ-350°C), were obtained between 0·05 and 4·5 eV by a combination of reflectance, transmittance and ellipsometric measurements. The refractive index at 0·15 eV decreases monotonically with increasing T s, or equivalently, with increasing density, and is 4·13±0·05 eV in the highest density films. The absorption edge is approximately exponential (102⪅α⪅104 cm−1) but shifts monotonically to higher energy and increases in slope with increasing T s. Similarly, the peak in e2 grows by about 10% and shifts by about 0·15 eV to higher energies, reaching a maximum of about 23 at 2·90±0·05 eV in the high density films. The peak in the transition strength ω2e2 occurs at 4·2±0·2 eV in all films, but increases in magnitude with increasing T s. The sum rules for n eff(ω) and e0,eff(ω) are evaluated for ▄ω⪅5 eV and vary monotonically with T s. These trends are neither compatible with Galeener...

Amorphous germanium III. Optical properties

The structure and growth of amorphous Ge films deposited at an angle of incidence ranging from 0° to 80° have been studied by electron microscopy and electron diffraction as a function of the film thickness, deposition temperature, deposition rate, and annealing temperature. Like crystalline films, the growth of amorphous films proceeds via random nucleation centers and is controlled by the small but finite adatom mobility. The small adatom mobility results in the formation of a large concentration of vacancies, vacancy clusters, voids, and associated dangling bonds in a‐Ge films. The adatom mobility is enhanced by the elevated deposition temperatures and oblique incidence and, consequently, the concentration of vacancy clusters and voids decreases. The formation of columns and decoration of structural defects associated with the cleaved NaCl single‐crystal substrates during the growth of a‐Ge films are a direct consequence of the enhanced adatom mobility. The observed thickness dependence of the resistiv...

Obliquely deposited amorphous Ge films. I. Growth and structure

The role that disorder plays in shaping the functional form of the optical absorption spectra of both amorphous silicon and amorphous germanium is investigated. Disorder leads to a redistribution of states, which both reduces the empirical optical energy gap and broadens the optical absorption tail. The relationship between the optical gap and the breadth of the absorption tail observed in amorphous semiconductors is thus explained.

M.-L. Theye

Papers

Influence of deposition conditions on the properties of amorphous germanium films