Germanium sulphide glass thin films have been deposited on CaF2 and Schott N-PSK58 glass substrates directly by means of chemical vapor deposition (CVD). The deposition rate of germanium sulphide glass film by this CVD process is estimated about 12 µm/hr at 500°C. These films have been characterized by micro-Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Their transmission range extends from 0.5µm to 7µm measured by UV-VIS-NIR and FT-IR spectroscopy. The refractive index of germanium sulphide glass film measured by prism coupling technique was 2.093±0.008 and the waveguide loss measured at 632.8nm by He-Ne laser was 2.1±0.3 dB/cm.
©2004 Optical Society of America
Chalcogenide glasses, especially sulphide glasses, are becoming more and more important for the fabrication of optoelectronic devices in part because of the high nonlinearity, strong photosensitivity and several unique properties they have [1–3]. The conventional method of fabricating a chalcogenide glass involves melting in a sealed ampoule. The elements in the form of pieces or powders, are placed in a quartz tube, which is then evacuated to low pressures, typically 10-3 Torr or lower, and sealed. The tube is placed in a furnace that rocks or oscillates it to homogenize the melt. Next, the ampoule is cooled, broken open and the glass then further processed to form, for example, thin films, optical fibre and optoelectronic devices. Thin films of chalcogenide glass can also be deposited by a number of methods including evaporation , sputtering , and ablation . These techniques in general suffer from difficulties associated with the incorporation of impurities or non-stoichiometry, which degrade the optical properties of the glass. Sol-gel techniques can be used for the fabrication of sulphide glass waveguides, but these materials are often contaminated by oxide impurities .
Chemical vapor deposition (CVD) has proved to be highly advantageous for the fabrication of ultra high purity silica glass fiber preforms. It would be desireable to find an analogous approach for the fabrication of chalcogenide materials and there has been considerable work devoted to this objective , although no suitable process has bee reported. Reaction between germanium chloride (GeCl4) and hydrogen sulphide (H2S) to form germanium sulphide (GeS2), for example, was reported to be unsatisfactory for the fabrication of planar or perform structures because of a low reaction rate and a low yield of deposited product. Another approach to form germanium sulphide films by plasma-enhanced CVD  was reported unstable films, easily oxidized in air, fabricated through the reaction between GeH4 and H2S.
The synthesis of chalcogenide glass using CVD techniques has not been widely reported. Therefore, with a source of the high purity germanium chloride (99.9999%) and the ability to melt in a reactive atmosphere of H2S available in our laboratory, we began to review the reaction between GeCl4 and H2S to form GeS2. A thermodynamic analysis (Section 2) indicates that this reaction should be favored and therefore preliminary tests were undertaken. The ability to fabricate thin films of chalcogenide is increasingly of interest, and the promise of practical planar integrated devices motivated this work .
2. Apparatus and experimental methods
The CVD apparatus for germanium sulphide deposition is shown in Fig. 1. Thin films of germanium sulphide glass form on a substrate through reaction between GeCl4 vaporized by means of a bubbler into argon carrier gas and H2S gas at temperature in the range of 450°C–600°C. This is a hot-wall CVD process under atmospheric pressure, in which a horizontal quartz tube reactor (25mm O.D. x 500mm long) is heated in a tube furnace. The reactive gas, H2S, and the carrier gas for GeCl4, argon, are delivered through the mass flow controllers (MFC) at the flow rate in the range of 50 ml/min–150 ml/min.
The thermodynamics and kinetics of the reaction between GeCl4 and H2S determine efficiency of the CVD process at a given reactant concentration and flow rate. No literature data are available for the kinetics of this reaction. A thermodynamics analysis reveals that formation of GeS2:
becomes increasingly favored at high temperature. The Gibb’s free energy of reaction (ΔGr), which determines the equilibrium constant, can be calculated from the difference between sums of the Gibb’s free energies of formation (ΔGf) of the products and reactants :
The equilibrium constant can be determined from ΔG=-RT ln(K) where R is the gas constant and T is the temperature in Kelvin. As the temperature is increased, the equilibrium constant becomes larger (Table 1), favoring the formation of GeS2. Non-stoichiometry is possible for germanium sulphide, which is not considered here. Nonetheless, as the temperature is increased, this analysis suggests that the reaction should also be increasingly favored for germanium sulphides with a stoichiometry that is close to 2.
Differential thermal analysis (DTA) of the germanium sulphide glass revealed a glass transition temperature of 456°C, a single crystallization event at 620°C and a melting temperature of 715°C. Therefore, the formation temperature of GeS2 in between 450°C and 600°C would form a more stable in glassy phase.
3. Characterization of germanium sulphide glass waveguides
Germanium sulphide glass has been successfully deposited on both CaF2 and Schott N-PSK58 glass substrates with a deposition rate estimated about 12 µm/hr at 500°C. Scanning electron microscope images (SEM) technique has been applied to study the germanium sulphide glass films deposited. From the cleaved edge of the germanium sulphide glass films on CaF2 and Schott N-PSK58 glass substrates are shown in Fig. 2. They show bubble and crack free thin films, free of any obvious inhomogeneity.
In this work, we use micro-Raman to characterize the composition of germanium sulphide glass and use UV-VIS-NIR spectrometer and FT-IR to study the optical transmission range of this glass. The Raman used is RENISHAW Ramascope which is equipped with a CCD camera. A 633nm He-Ne laser was used to excite the sample and the Raman shift spectrum was measured from 1000cm-1 to 100cm-1 with a resolution of 1cm-1. The Raman spectrum is shown in Fig. 3 from which we can verify the main GeS4 tetrahedra band at 342cm-1 and 374cm-1 and band due to short S-S chains between GeS4 tetrahedra at 434cm-1. Comparison of the measured spectrum with that in reference  estimated germanium glass with a sulphur to germanium ratio of 2±0.2. Furthermore, from this Raman spectrum, an amorphous phase of the germanium sulphide has been demonstrated.
The optical transmission range of germanium sulphide glass fabricated by CVD process has been studied by Varian Gary 500 Scan UV-VIS-NIR Spectrophotometer at the resolution of 1nm and Perkin-Elmer spectrum 2000 FT-IR at the resolution of 4cm-1. The germanium sulphide glass was prepared by collecting the germanium sulphide powders from the CVD process and melting them by the sealed- ampoule method. The results shown in Fig. 4 reveal a S-H absorption band at about 4µm and O-H absorption at about 2.7 µm, which is typical of many chalcogenide glasses. With knowledge of the extinction ratios for these impurities , we have estimated the concentration of these impurities at ~3ppm and ~37ppm respectively. With further impurity reduction, the useful transmission range of germanium sulphide glass is from 0.5 to 7µm.
The X-ray diffraction (XRD) technique is used to investigate the structure of the germanium sulphide glass film. The result, shown in Fig. 5, indicates no significant crystalline peak in comparison to the structure of the crystalline GeS2 below for reference. From this pattern, we can certify the glass film is amorphous and is of glassy phase.
The refractive index of germanium sulphide glass film has been characterized by the prism coupling technique shown in Fig. 6. The prism used in the measurement was rutile with a refractive index (np) of 2.584 for light polarized to excite the TM mode.
A least squares fit on this data than allowed the calculation of the refractive index and film thickness, which were 2.093±0.008 and 1.83±0.08µm respectively. The results also agree with that in the reference .
There has been no significant visible change in the films over one year, indicating some stability, however further tests are undertaken to investigate any long term oxidation.
4. Waveguide fabrication
To fabricate a series of channel waveguides, a germanium sulphide glass film was patterned by the photo-lithography and rib waveguides formed by argon ion-beam etching techniques (Fig.7).
In the photolithography process, we use a positive Shipley S1813 photoresist and Puddle MF319 developer. An Ar ion-beam milling instrument (OXFORD and EDWARDS) was used to etch the patterned film. The Ar ion-beam is non-selective and etched the photoresist and germanium sulphide glass at about the same rate of 30nm/min. The thickness of the photoresist patterned on the germanium sulphide glass was about 1µm, therefore a run time of 30mins was used. The residual photoresist was removed with acetone, followed by a rinse in D.I. water. As shown in Fig. 7, well defined rib structures with a width of 5µm have been achieved. The waveguide loss was measured by fibre coupling technique with a He-Ne laser at 632.8nm to be 2.1±0.3dB/cm. Further measurements at longer wavelengths are in progress but these first results indicate the CVD process is promising for germanium sulphide waveguide fabrication.
Germanium sulphide glass waveguides on some selected substrates have been successfully fabricated by chemical vapour deposition (CVD). The properties of the waveguides have been studied and the light guidance of the waveguide has been demonstrated. These results show this fabrication technique has a great potential in optoelectronics, particularly as waveguides for optical integrated circuits applications. We are currently extending this work to deposition on semiconductor substrates.
The authors would like to acknowledge the technical assistance of Mr. John Tucknott, Mr. Neil Fagan and Mr Kenton Knight. Also, for his work on electronic control, Mr. Trevor Austin is acknowledged. This work was funded by the Engineering Physical Sciences Research Council.
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