Germanium Carbide Films Research Articles

Effect of substrate temperature and radio frequency power on compositional, structural and optical properties of amorphous germanium carbide films deposited using sputtering

Fabrication of GexC1−x has been a big challenge because of the solubility of C in Ge. Only a small percentage of GeC bonds (11.6%) have been introduced so far. In this work, a-GexC1−x with GeC content up to 21% has been fabricated with 50W RF power at 250°C by reactive sputtering methods. The effects of the radio frequency power and substrate temperature on the yield of GeC were analysed in detail. The GeC percentage by volume was found to first increase and then decrease with increasing substrate temperature. Introduction of C into the Ge matrix seems to tune the optical bandgap over a range of 2.7eV to 1.0eV depending on the combination of substrate temperature and radio frequency power.

Increasing sp3 hybridized carbon atoms in germanium carbide films by increasing the argon ion energy and germanium content

We have prepared germanium carbide (Ge1−xCx) films on Si(0 0 1) by radio frequency (RF) reactive sputtering a pure Ge(1 1 1) target in a CH4/Ar mixture discharge, and found that the sp3 hybridized carbon atoms in the Ge1−xCx film can be significantly increased in two ways. One is by increasing the Ge content via increasing the RF power during the film deposition, which can lead to a transition from sp2 C–C to sp3 C–Ge bonding in the film. Another is by increasing the Ar ion energy in a discharge Ar/CH4 gas by applying the negative bias voltage, which plays an important role in inducing the compressive stress in film. We find that when the compressive stress increases above a critical value of 2.2 GPa, an abrupt transition from sp2 C–C to sp3 C–C bonding occurs in the Ge1−xCx film, which is a consequence of energy minimization.

Annealing effects on the bonding structures, optical and mechanical properties for radio frequency reactive sputtered germanium carbide films

The effects of thermal annealing in vacuum on the bonding structures, optical and mechanical properties for germanium carbide (Ge1−xCx) thin films, deposited by radio frequency (RF) reactive sputtering of pure Ge(111) target in a CH4/Ar mixture discharge, are investigated. We find that there are no significant changes in the bonding structure of the films annealed below 300°C. The fraction of Ge–H bonds for the film annealed at temperatures (Ta) above 300°C decreases, whereas that of C–H bonds show a decrease only when Ta exceeds 400°C. The out-diffusion of hydrogen promotes the formation of Ge–C bonds at Ta above 400°C and thus leads to a substantial increase in the compressive stress and hardness for the film. The refractive indices and optical gaps for Ge1−xCx films are almost constant against Ta, which can be ascribed to the unchanged ratios of Ge/C and sp2-C/sp3-C concentrations. Furthermore, we also find that the excellent optical transmission for an antireflection Ge1−xCx double-layer film on ZnS substrate is still maintained after annealing at 700°C.

Effects of radio frequency power on the chemical bonding, optical and mechanical properties for radio frequency reactive sputtered germanium carbide films

Germanium carbide (Ge1−xCx) films have been prepared by radio frequency (RF) reactive sputtering of a pure Ge(111) target in a CH4/Ar mixture discharge, and their composition, chemical bonding, optical and mechanical properties have been investigated as a function of RF power. The relationship between the chemical bonding and the optical and mechanical properties of the Ge1−xCx films has been explored. The results show that the refractive index of Ge1−xCx films increases from 1.9 to 3.2 and the optical gap decreases from 1.9 to 1.0 eV as RF power increases from 80 to 250 W, which is due to an increase in the germanium content with increasing RF power. Also, it is found that the hardness of Ge1−xCx films increases with increasing RF power, which can be attributed to an increase in the fraction of sp3 Ge–C bonds at the expense of the sp2 C–C bonds in the Ge1−xCx films.

Effects of the chemical bonding on the optical and mechanical properties for germaniumcarbide films used as antireflection and protection coating of ZnS windows

Germanium carbide (Ge1−xCx) films have been prepared by RF reactive sputtering a pure Ge(111) target at different flow rate ratiosof CH4/(CH4+Ar) in aCH4/Ar mixturedischarge, and it has been found that the composition, chemical bonding, optical and mechanical propertiesof Ge1−xCx films are remarkably influenced by the flow rate ratio ofCH4/(CH4+Ar). The effects of the chemical bonding on the optical and mechanical properties of theGe1−xCx films have been explored. In addition, an antireflectionGe1−xCx double-layer coating deposited on both sides of the ZnS substratewafer has been developed for application as an infrared window. It isshown that the transmittance in the wavelength region between 8 and12 µm and the hardness of the ZnS substrate have been significantly improved by the double-layercoating.

Nanocrystalline Germanium and Germanium Carbide Films and Devices

AbstractNanocrystalline Ge and its alloys with C are potentially useful materials for solar cells, thin film transistors and image sensors. In this paper, we discuss the growth and properties of these materials using remote, low pressure ECR plasma deposition. The materials and devices were grown from mixtures of germane, methane and hydrogen. It was found that high hydrogen dilutions (>40:1) were needed to crystallize the films. Studies of x-ray spectra revealed that the grains were primarily <220> oriented. The grain size was a strong function of hydrogen dilution and growth temperature. Higher growth temperatures resulted in larger grain size. High hydrogen dilution tended to reduce grain size. These results can be explained by recognizing that excessive amounts of bonded H can inhibit the growth of <220> grain, which is the thermodynamically favorable direction for grain growth. Grain sizes as large as 80 nm were obtained in nc-Ge. Addition of C reduced the crystallinity. Mobility and carrier concentrations in nc-Ge were measured using Hall effect. Mobility values of ˜5cm2/V-sand carrier concentrations of ˜1x1016/cm3were obtained in larger grains. p+nn+ devices were fabricated on stainless steel substrates and compared with similar devices deposited in nc-Si:H. It was found that the voltage decreased and current increased in nc-Ge devices, in comparison with devices in nc-Si:H. Addition of C to Ge devices increased the open circuit voltage and shifted the quantum efficiency to larger photon energies, as expected.

Structure and Properties of Germanium Carbide Films Prepared by RF Reactive Sputtering in Ar/CH4

Germanium carbide ( Gex C1-x ) films have been prepared by RF reactive sputtering at various mixtures of argon and methane from a germanium target. The compositions, structures and properties of Gex C1-x films have been investigated using Auger electron spectroscopy (AES), Visible near-infrared photometer, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The AES analyses show the atomic ratio (Ge/C) of the films decreased with increasing the gas flow ratio CH4/( Ar+CH4 ). The XRD and SEM results indicate that the films were generally smooth and featureless, and had amorphous structure. The refractive index of Gex C1-x films can be varied between 1.8 and 4.3 by changing the gas flow ratio. The hardness values of the films grew with the augmentation of the amount of carbon content in the films, and were higher than those of IR optical material Ge and ZnS. It can be predicted that Gex C1-x films possess excellent abrasion resistance.

Asymmetric Electron Spin Resonance Signals in Hydrogenated Amorphous Germanium Carbide Films

Asymmetric Electron Spin Resonance Signals in Hydrogenated Amorphous Germanium Carbide Films

Electron energy-loss studies on radio frequency sputtered a-Ge1−xCx:H films

Valence-band and core level electron energy-loss spectroscopy (EELS) were used to study rf sputter deposited hydrogenerated amorphous germanium carbide films (a-Ge1−xCx:H, 0≤x≤1). The optical gap of the films ranged from 0.8–1.0 eV in a-Ge:H to 1.2–2.3 eV in a-C:H. Valence-band EELS of clean sputter etched a-Ge and a-Ge:H films (x=0), as compared to that of Ar+ sputter etched Ge(111) and hydrogen covered single crystals of Ge, respectively, indicated preferential sputtering of hydrogen by Ar ions, in contrast to recent observations on rf sputter deposited a-Si:H, where Ar+ etching did not cause complete depletion of hydrogen from the surface. In a-Ge1−xCx:H alloy films (0<x<1) loss peaks related to carbon and Ge–C bonds were identified. Auger analysis of as prepared a-C and a-C:H films (x=1) indicated relatively clean surfaces. Typically, only 1%–2% oxygen was found on the surfaces. EELS experiments indicated graphitelike surface structures. In addition, the hydrogen bulk content had no influence on the spectra, suggesting that the surfaces of the a-C:H films were hydrogen depleted. The Auger carbon line shape further confirmed this assumption.