DC conductivity of amorphous germanium

A critical review of the phonon-assisted hopping theory of the dc conductivity σ is given. It is argued that the current theories either contain uncontrollable approximations, or they yield the result of the same order of magnitude as the approximations used or they may also lead to temperature dependence of σ other than that described by the Mott law. For the simplest usually accepted model of amorphous Ge (a-Ge), the low-temperature asymptote of σ is obtained in the form of a power law. In the experimental part of the work, careful measurements from 300 down to 8 K are reported, both for the pure and the gold-doped a-Ge films. Since the results of such measurements yield a straight line in a wide interval of temperature both in the log σ versus $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ and log σ versus log T scale, the power law appears to be equally well justified as the Mott law, both from the point of view of theory and that of experiment.     

Electrical conductivity and crystallization in amorphous germanium

A simple model is presented which interprets conductivity variations due to crystallization in amorphous layers and allows the determination of the growth rate of crystallization. The validity of this model is demonstrated in the case of germanium.     

Magnetoresistance in amorphous germanium

The magnetoresistance of evaporated amorphous germanium films ranging in thickness from 200 Å to several microns has been measured in the temperature range 77 K < T < 300 K for magnetic fields ranging from 5 G to 100 kG. The results are in general agreement with those of Mell and Stuke. Various models which attempt to account for the magnetoresistance are discussed, but it is concluded that the origins of this effect are not understood at this time. The magnetoresistance measured at 77 K in the presence of a high electric field (45 000 V/cm) shows the same behavior as obtained for low electric fields.     

High field effects in amorphous germanium

Measurements of high field current have been made on amorphous Ge (a-Ge) films over the temperature range from 100 to 300 K. Non-ohmic conduction in a-Ge occurs at electric fields greater than 1–2 × 10⁴ V/cm. Field dependence of the conductivity has been explained in terms of the enhanced emission probability of carriers from the screened coulombic trap centers. Assuming the optical dielectric constant for a-Ge films, the screening length of the trap centers and the density of states at the Fermi level are estimated to be 12 Å and ～6.1 × 10²⁰ cm^?3 eV^?1, respectively.     

Ionization enhanced crystallization in amorphous germanium

The effect of high-energy electron irradiation upon the kinetics of crystallization of amorphous germanium layers has been studied, using conductivity measurements, during isochronal and isothermal warm-up. An increase in the growth rate of crystallization, induced at the surface or in the bulk, has been observed. It is suggested that this increase is the consequence of the recombination of electron-hole pairs created by the irradiation.     

Disorder and optical absorption in amorphous silicon and amorphous germanium

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.     

Crystallization of amorphous germanium films

Kinetics of isothermal crystallization has been studied in the temperature range from 375 to 525°C. The kinetic curves are obtained and the rate of isothermal transformation of amorphous films into crystalline ones has been determined. Using experimentally determined kinetic curves the stability diagram of the amorphous films has been plotted in the temperature range from 400 to 525°C. The value of effective activation energy has been defined.     

The crystallization of amorphous germanium films

Germanium thin films were prepared in an amorphous form by vacuum deposition onto room temperature fused silica substrates. The amorphous—crystalline transition was studied as a function of time and temperature by measuring the optical transmission near 0.65 μm, where the absorption constant is most sensitive to the phase transformation. At a fixed temperature, the time for half the volume of the sample to become crystallized was found to be consistent with the relation t_c = τ exp(E₀/kT), with an activation energy E₀ = 2.96 eV.     

On the structure of amorphous germanium and silicon

The simple microcrystallite model is used to calculate the diffraction and radial distribution functions for a variety of tetrahedrally coordinated crystal structures: diamond, wurtzite, Ge III and Si III — two high-pressure polytypes of Ge and Si — and two clathrate structures based on pentagonal dodecahedral units. Comparison with data for sputtered amorphous Ge suggests that the simple microcrystallite model is inadequate to fit diffraction data. A statistical or combination microcrystallite model may be more promising. Recent electron microscopic investigations of Rudee and Howie are also discussed.     

The effect of nitrogen doping on amorphous germanium

The effects of nitrogen doping on the electrical and optical properties of amorphous germanium are investigated. It is found that within the low nitrogen concentrations that cause no appreciable change in the optical energy gap, the room temperature conductivity and the B coefficient in the optical absorption show a maximum at the same nitrogen concentration. This behavior is interpreted by a delocalization of the electronic states in the conduction band due to the nitrogen incorporation.     

Dark conductivity in amorphous undoped silicon films

The temperature dependence of the dark conductivity was investigated in amorphous undoped silicon films deposited by glow-discharge in a SiCl₄H₂ mixture. Different transport processes were indentified according to the investigated temperature range. The dependence of the dark conductivity was also examined as a function of some deposition parameters. The experimental results are discussed in terms of the two-phase structure of the film.     

Electronic structure of the wrong-bond states in amorphous germanium sulphides

First-principles calculations of the electronic structures of the wrong bonds were performed for amorphous germanium sulphides in order to explain their compositional dependence. Model cluster calculations of the density of states using a set of geometry similar to the crystalline-GeS₂ coordination can reproduce the peak structure of the experimental valence band photoemission spectra. The bonding and anti-bonding states of the covalent Ge-S bonds form valence and conduction band respectively, and the top of the valence band is occupied with S 3p lone-pair states. The bonding states are modified by S-S bonds and the anti-bonding states are modified by Ge-Ge bonds, mainly through their hybridization with the wrong-bond states between p-orbitals. The lone-pair states do not interact either of them to form a different band, and obscure the modification induced by the S-S wrong bonds. Therefore, we can conclude that the narrowing of the bandgap with increasing Ge content from GeS₂ composition is due to that of the conduction-band bottom with increasing germanium wrong bonds, though the narrowing with increasing S content is moderate due to the presence of the lone-pair states at the valence-band top.     

Calorimetric,diffraction and microscopic examination of evaporated amorphous germanium

Dynamic calorimetric and thermal gravimetric measurements revealed that evaporated, amorphous Ge adsorbed up to 3 wt % H₂O when exposed to the atmosphere. Desiccation removed one-half; however, the remainder was removed only by heating to 450°C which also completely recrystallized the material. The heat of crystallization was found to be, ΔH_c = 2.6 ± 0.4 kcal/mol.Electron microscopic examination revealed ≈100Å void network as found by others with heterogeneous crystallization as samples were annealed at ≈200°C. During the annealing and crystallization, the normally forbidden (222) electron diffraction reflection was frequently observed. We interpret this as being due to (111) twinning which is evidence of wurtzite-type bonding in the original evaporated, amorphous Ge material.     

Transport properties of amorphous germanium prepared by the glow discharge technique

The paper deals with conductivity, thermoelectric power and field effect measurements on amorphous Ge specimens prepared by the decomposition of germane gas in a rf glow discharge. Substrate temperatures T_d of 300, 400 and 500 K were used during deposition. The sign of the thermoelectric power S is negative throughout the temperature range investigated (200–500 K). Above 300 K, the conductivity activation energy in specimens prepared at T_d = 500 K lies between 0.40 and 0.43 eV; it is equal to the gradient of the S versus 1/T curves, suggesting transport in the extended electron states. Below room temperature there is an increasing contribution in all specimens from electron hopping transport in localized states lying about 0.25 eV below ?_C. Both conductivity and thermoelectric power results can be interpreted satisfactorily in terms of these two current paths. Hopping at the Fermi level has not been observed. The preliminary field effect measurements indicate that, as in amorphous Si, ?_f lies near a density of state minimum. The density of states at ?_f is appreciably higher than that in similarly prepared Si specimens.     

Effective local environment model for vibrational states in pure amorphous germanium and silicon

A detailed comparison between the measured infrared absorption (IR) spectra of pure amorphous germanium and silicon and the calculated density of vibrational states (DVS) and IR spectra for several models shows the need for an improved model of this system. A small (17 atom) crystalline cluster model is proposed, in which the imbedding of the cluster in the bulk amorphous medium is simulated by nearest-neighbor forces on the surface atoms of the cluster. The surface force constants are adjustable parameters of the model. A DVS is obtained which has the distinct three-peaked form of the measured IR, and systematic variation of the surface parameters show the range in which these may be adjusted to achieve precise agreement with the peak frequencies of the observed IR.