Structural peculiarities of amorphous germanium films

The number of nuclei grown as a function of time, overpotential and eletrolyte concentration have been investigated experimentally during the electrodeposition of Hg from an aqueous solution of Hg₂(NO₃)₂ onto a plane structureless platinum electrode and a hemispherical single-crystal electrode. It has been established that: (i) at constant overpotential after an induction period the number of nuclei grows linearly with time and reaches a saturation value after a sufficient time has elapsed; (ii) the higher the overpotential the shorter the induction period and the higher both the steady state nucleation rate and the saturation nucleus density; (iii) the smaller the electrolyte concentration the lower the nucleation rate, the longer the induction period and the larger the saturation nucleus density. It is shown that the time dependence of the number of nuclei can be described by an equation derived by assuming that nuclei are formed on active energetically preferred sites on the electrode surface, and that the saturation nucleus density is determined by the deactivation of the active centres by overlapping nucleation exclusion zones. Experimental data confirming the idea of Mutaftschiew and Toschev that the active centres are defects in the oxide layer covering the platinum are presented.     

Electrical characterization of amorphous germanium dioxide films

Amorphous germanium dioxide (GeO₂) films have been deposited by electron beam evaporation onto different substrates including glass, SnO₂ conducting glass, evaporated gold and n-type silicon in order to examine the electrical behaviour of GeO₂ in metal/insulator/metal (MIM) and metal/insulator/semiconductor (MIS) structures. In MIM structures the as-deposited films are strongly influenced by electrode barriers but heat treatment at 600 K induced ohmic behaviour. The dielectric response of the films in the frequency range 0.1–100 kHz and the temperature range 180–350 K showed that the dielectric constant at 300 K was 9 and was virtually independent of frequency, while the a.c. conductivity follows the relation σ ∞ ω^s, where s is temperature dependent. Good agreement with a classical electronic hopping model is obtained. In MIS structures, GeO₂ on silicon gives rise to heterojunction behaviour at low voltages while, at higher voltages, the d.c. conduction is bulk dominated and exhibits space-charge-limited conduction. The dielectric response of MIS structures is strongly influenced by the depletion capacitance at the interface between GeO₂ and silicon.     

Annealing and crystallization of amorphous germanium thin films

The isochronal and isothermal annealing of the room temperature electrical resistivity of vacuum-deposited a-e thin films was studied for different annealing temperatures up to about 500°C. By analysing the annealing kinetics, it is possible to show that the crystallization process can be characterized by a single activation energy of 3.5 eV and the nucleation process by an activation energy of 3 eV for different coating conditions. It is proposed that the crystal growth takes place by diffusion of extended divacancies to the crystalline-amorphous interface and by subsequent rearrangement of the relaxed atoms fit the crystalline matrix.     

Effect of annealing and oxygenation on the DC conductivity of amorphous germanium

Thin films of amorphous germanium were deposited in an oxygen atmosphere.dc conductivity results are interpreted considering the possibility of the formation of Ge-O bonds. The density of states was determined. Results of conductivity are interpreted using the Davis-Mott model. Change in conductivity in annealed films of V-a-Ge and O-a-Ge is also reported.     

The kinetics of transformation in amorphous germanium alloy films

The kinetics of the amorphous-to-crystalline transformation were studied in amorphous germanium (a-Ge) and its alloys with aluminium, silicon, indium, copper, iron, nickel, gallium and tin by using isochronal and isothermal annealing of the room temperature resistivity, differential thermal analysis and electron microscopy measurements. It was shown that the transformations in a-Ge and a-Ge alloy films are nucleation and growth controlled. The activation energy of 3.0 eV obtained for the amorphous-to-crystalline transition in a-Ge was found to decrease on alloying with metals, and the decrease depended on the nature of the alloying element.     

Band gap profiles of intrinsic amorphous silicon germanium films and their application to amorphous silicon germanium heterojunction solar cells

Intrinsic amorphous silicon germanium (i-a-SiGe:H) films with V, U and VU shape band gap profiles for amorphous silicon germanium (a-SiGe:H) heterojunction solar cells were fabricated. The band gap profiles of i-a-SiGe:H were prepared by varying the GeH₄ and H₂ flow rates during the deposition process. The use of i-a-SiGe:H with band gap profile in an absorber layer for a-SiGe:H heterojunction solar cells was investigated. The solar cell using a VU shape band gap profile shows a higher efficiency compared to other shapes. The highest efficiency obtained for an a-SiGe:H heterojunction solar cell using the VU shape band gap profile technique was 9.4% (V_oc = 0.79 V, J_sc = 19.0 mA/cm² and FF = 0.63).     

Thickness dependence of d.c. conductivity of amorphous Zn3P2 films

Journal of Materials Science Letters -     

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.     

Abnormal morphology of amorphous germanium films in contact with palladium

The fractal and dense branching morphologies of amorphous germanium films in contact with palladium have been investigated by TEM. The experimental results suggest that the production of fractal morphology in Pd/a-Ge bilayer films was easier than that in a-Ge/Pd bilayer films. An island-like fractal morphology can be formed at the step and crevice hole areas annealed at higher temperatures. It is difficult for co-evaporated Pd-Ge films to realize fractal morphology. The formation of fractal morphology can be explained by a RSN model. The SAED patterns suggest that the dense branching morphology cannot be completely demonstrated by the Pd, Ge, Pa₂Ge, PdGe and Pd₂₅Ge₉, maybe it is a new phase.     

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 (=1.06 m, 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.     

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.     

Dynamic conductivity of amorphous nanogranular films in the microwave frequency range

The electric conductivity of amorphous nanogranular composite films has been experimentally studied on direct current and under microwave reflection conditions. It is established that the dynamic conductivity exceeds the static value by two to four orders of magnitude. Suggestions concerning the nature of this difference are formulated.     

A study of the optical properties of amorphous thin films of germanium and silicon monoxide

Amorphous films of Ge-SiO have been co-evaporated and some of their optical properties are reported. The optical constants have been measured and estimated. At the high absorption end of the absorption edge, an equation due to non-direct transitions inK-space is found to match the optical absorption data. The variation of the optical band gapE _opt with film composition is reported. The infrared spectrum of a mixed layer is presented and a simple conclusion is drawn.