Properties Of Amorphous Germanium Films Research Articles

Shifting of the thermal properties of amorphous germanium films upon relaxation and crystallization

The modification of the thermal conductivity and melting temperature of unrelaxed amorphous Ge films on Si substrates upon laser-induced relaxation and crystallization is presented. Real-Time Reflectivity (RTR) measurements are used to determine experimentally both the melting threshold and the melt durations, and the finite element method is used to simulate the laser-induced heat-flow process. A thermal conductivity ofk=0.010 W dem K is determined for the unrelaxed material by fitting the experimental melting thresholds of unrelaxed films of different thicknesses. A similar procedure applied to the amorphous relaxed and crystallized materials lead to a shift to higher values of both the thermal conductivity and the melting temperature. In order to achieve a good fit of the experimental melt durations, it was necessary to assume a large degree of undercooling prior to solidification. The role of undercooling in the solidification process is finally discussed in terms of its dependence on the faser energy density and the high thermal conductivity of the substrate.

Shifting of the thermal properties of amorphous germanium films upon relaxation and crystallization

Shifting of the thermal properties of amorphous germanium films upon relaxation and crystallization

Influence of substrate temperature on the electrical and optical properties of amorphous germanium films

The influence of substrate temperature on electrical and optical properties of the amorphous germanium films deposited under well-defined conditions has been investigated. DC electrical conductivity in the temperature range of 80–573°K has been measured. In the low temperature region Mott’sT −1/4 law of conductivity is obeyed. The estimated values ofT 0 andN show significant decrease with change inT s in steps of 50°K. Similar results are seen in annealed films. The values of activation energy and optical energy increase withT s.

Effect of laser irradiation on the electrical properties of amorphous germanium films

Amorphous films of germanium were grown using a vacuum evaporation technique, on glass substrates kept at room temperature. As-grown films were irradiated with Q-Switched Nd-YAG laser pulses (λ=1.06 μm, 20nsec, 10 to 50Jcm−2). The d.c. conductivity measurements were made in the temperature range 77 to 300 K. It was observed that the effect of laser irradiation was similar to the effect caused by the thermal annealing of the films. The d.c. conductivity data were analysed in the light of Mott's theory of a variable range hopping conduction process.

Effect of Substrate Temperature on Electrical Properties of Amorphous Germanium Films

Electrical properties of a series of germanium films deposited onto glass substrate held at temperature between 100 and 300°C have been investigated. There is no significant difference in the structural and electrical characteristics of the amorphous films deposited at the substrate temperature lower than 230°C. Otherwise, the results obtained with the substrate temperature higher than 230°C are as follows: (a) the degree of the disorder in structure decreases and crystal orientation is observed, (b) the resistivity decreases, (c) the slope of the log resistivity against inverse temperature (1/T) decreases, (d) the slope of the log resistivity against 1/T1/4 changes from linear to non-linear, and (e) the slope of the log current against square root of the electric field increases. These results can be explained by considering of the diffused-localised states formed by the disorder near the valence band edge.

Optical and Photoelectrical Properties of Amorphous Germanium Films*

We studied the photoemission process in films of amorphous Ge deposited on silica substrates. We measured, for both polarizations and for several angles of incidence, with direct and back illumination (through the substrate), the reflectance, the transmittance, and the photoemissive yield of each film without exposure to air. We analyzed the possibility of deducing with minimum error, from the experimental results, the index of refraction, the thickness, and the escape probability of photoexcited electrons and we verified the validity of the model used for computation.

Influence of annealing on the optical properties of amorphous germanium films

The influence of annealing on the optical, electrical and structural properties of thin amorphous Ge films deposited under well-defined conditions has been investigated. The results below the onset of recrystallization are interpreted as due to a progressive elimination of defects and an evolution of the films towards an “ideal” amorphous state. Various mechanisms are discussed in relation with various structural models.

Influence of deposition conditions on the properties of amorphous germanium films

The optical and electrical properties of thin films of amorphous germanium are investigated as a function of the conditions of deposition. The results are compared with the results of annealing experiments and interpreted in terms of varying amounts of imperfections in the amorphous films.

Optical Properties of Amorphous Germanium Films

The optical constants of amorphous Ge films formed under well-defined conditions have been determined in the 0.1-25.0-eV spectral range. In the 0.1-1.8-eV range they were determined by analysis of precise reflectance and transmittance data ($\mathrm{RT}$), and in the 0.1-25.0-eV range by a Kramers-Kronig analysis of normal-incidence reflectance data. Both analyses gave the same results in the region of overlap. The absorption edge was found to be quite sharp (0.06 eV) and to occur usually at a photon energy of about 0.6 eV. The position of the edge was normally about 0.2 eV lower than the direct edge in crystalline Ge. No evidence was found for either the spin-orbit split valence band associated with crystalline Ge or a tailing and/or large number of states in the forbidden region. The smallest nonzero value of the absorption measured on the low-energy side of the absorption edge was about 10 ${\mathrm{cm}}^{\ensuremath{-}1}$. There was no evidence for free-carrier absorption further in the infrared. At 0.1 eV, the index of refraction was 3.99 \ifmmode\pm\else\textpm\fi{} 0.04 as determined by the $\mathrm{RT}$ analysis. This value was in excellent agreement with the value of 4.00 derived from the zero-frequency dielectric constant, which has been calculated using the sum rule. This is also the value determined for crystalline Ge in the infrared. Reflectance data for amorphous Ge films deposited and measured in ultra-high vacuum (in situ) are reported for the region 2.0-11.8 eV. The density, determined by weighing films of known thickness, was 12-15% less than the density of crystalline Ge.

Structural, Electrical, and Optical Properties of Amorphous Germanium Films

Measurements of the ac and dc resistivity in the temperature range of 77-750\ifmmode^\circ\else\textdegree\fi{}K and of the optical transmission and reflectance have been made on amorphous Ge films of 1000-\AA{} to 7-\ensuremath{\mu} thickness prepared by thermal evaporation. The resistivity of amorphous Ge films at room temperature is \ensuremath{\sim}100\ensuremath{\Omega} cm, and increases with decreasing temperature. The conduction is thermally activated, with activation energy decreasing continuously from 0.5 eV at 600\ifmmode^\circ\else\textdegree\fi{}K down to 0.069 eV at 77\ifmmode^\circ\else\textdegree\fi{}K. At 450\ifmmode^\circ\else\textdegree\fi{}C, the amorphous \ensuremath{\rightarrow} crystalline transformation is observed and is accompanied by an irreversible decrease of resistivity by a factor of \ensuremath{\ge}20. The ac resistivity of amorphous films is found to decrease with frequency $\ensuremath{\omega}$. At high frequencies, the resistivity varies as ${\ensuremath{\omega}}^{\ensuremath{-}n}$, where $n$ lies between 0.5 and 1, depending on temperature. The resistivity decreases with applied field beyond the Ohmic regime. At 77\ifmmode^\circ\else\textdegree\fi{}K, the current increases exponentially with the square root of the field for fields $\stackrel{\mathrm{\ensuremath{\backsim}}}{>}$ 2.5\ifmmode\times\else\texttimes\fi{}${10}^{3}$ V/cm. The optical absorption coefficient is found to vary as exp ($\frac{h\ensuremath{\nu}}{0.14}$ eV), in the energy range of 0.6-1.24 eV. Below 0.6 eV, the absorption coefficient falls rapidly down to a value of \ensuremath{\sim}60 ${\mathrm{cm}}^{\ensuremath{-}1}$ at 0.53 eV. The dc and ac electrical properties may be understood in terms of a mechanism of conduction by hopping of the carriers in the localized states (near the "fuzzy" band edges) caused by the fluctuation potential in the disordered lattice. The exponential decay of the opticalabsorption coefficient with energy near the absorption edge and the presence of a red-shifted absorption edge are consistent with the crystalline energy-band diagram modified by the presence of acceptor states due to vacancies, by tailing, and by localization of states near the band edges due to disorder. The sharpness (which is comparable to that in single-crystal Ge) of the amorphous absorption edge is, however, not compatible with the "tailing" concept.