James A. Savage

Monitoring the quality of diamond films using Raman spectra excited at 514.5 nm and 633 nm

Abstract A relative Raman scattering cross-section has been experimentally determined for diamond and non-diamond carbon spectra excited by argon ion 514.5 nm radiation. This has been used to arrive at a semi-quantitative evaluation of diamond film quality. The results are compared with those obtained using helium-neon excitation at 633 nm.

The potential of Ge-As-Se-Te glasses as 3–5 μm and 8–12 μm infrared optical materials

Abstract A region of the Ge-As-Se-Te glass phase diagram is identified as suitable for exploitation. The potential range of optical properties of these glasses is established and measured thermal and optical data on selected glasses is presented.

Infrared optical glasses for applications in 8–12-μm thermal imaging systems

Measured data are presented on glass transition temperature, thermal expansion coefficient, and refractive index for glasses in the Ge-As-Se system. The range of reciprocal dispersive power between 8 microm and 12 microm of 110-159 for these glasses is discussed in relation to their basic physical properties, and it is shown that useful compositions for bulk synthesis can be identified. A brief account is given of the preparation techniques, which enable glass to be made with an oxide content of the order of 1 ppm wt as measured by gamma photon activation analysis. The influence of oxide impurity on the transmission is shown for the three glasses Ge2O-Se80, As40-Se60, and Ge30-Asl5-Se55.

Diamond coatings for IR window applications

Abstract Diamond layers have been deposited by microwave-assisted plasma chemical vapour deposition on germanium and zinc sulphide, for infrared window applications. Advances have been made which reconcile the difficult requirements of coping with severe thermal expansion mismatch, ensuring mechanical integrity, strength, and resistance to erosion, while maximising optical transmission. After an interface preparation technique and the deposition of an interlayer of appropriate thickness and refractive index, a diamond layer 5–10 μm thick is deposited at 600°C or below. The interlayer protects the zinc sulphide from erosion in the aggressive H2 plasma. The improved mechanical properties of the composite structure have been confirmed by the higher rain impact damage threshold velocity of over 300 ms−1 for germanium, and the consequences for particle impact studies are presented. The resulting coated structure is shown to be near the theoretical optical transmission limit. Modelling of the optical properties of the structure is also presented.

IR attenuated total reflectance studies of d.c. biased growth of diamond films

Abstract The IR spectra of polycrystalline diamond films grown under negative d.c. biased conditions were recorded using both conventional transmission and total internal reflectance spectrometry. The films were grown by microwave enhanced plasma deposition on singlecrystal silicon using methane-hydrogen mixtures at powers of up to 0.8 kW. IR spectra in the carbon-hydrogen stretching region were recorded over the wavenumber range 4800–400 cm−1 using Fourier transform techniques. The profiles of the IR bands of the biased and non-biased samples showed the presence of differing carbon-hydrogen groupings. In the non-biased samples only hydrogen bound to sp3 hybridized carbon was detected. In contrast, the biased films showed the presence of sp2 hybridized C-H groupings for films deposited with a 10% methane in hydrogen mixture. The spectroscopic evidence could not distinguish between the sp2 C7 H derived from either graphitic or =CH containing phases. When growth at normal conditions, 0.5% methane in hydrogen, was performed after biased enhanced nucleation, the overall hydrogen content of the film remained similar although the distribution of chemically bound hydrogen amongst the sp3 hybridized CH groupings altered significantly.

Diamond protective coatings for optical components

Abstract The use of polycrystalline diamond films as protective coatings on IR materials has been highlighted as a major application of CVD diamond technology. However, those materials which would benefit most from diamond coatings, such as ZnS and Ge, possess vastly different thermomechanical properties. The severe technological problems which exist have prevented commercialization to the levels achieved by heat sinks and cutting tools. We have been working for some time on the direct deposition of diamond onto protected ZnS windows and have extended our process to include other IR materials. By employing void-free protective interlayers we have produced diamond films up to 1 μm thick on planar interlayers on ZnS. Lithographic substrate patterning has been used to exceed this barrier and increase the achievable thickness towards the levels required for true protection from water drop or particulate erosion. Deposition onto sapphire substrates has permitted considerable progress in overcoming mechanical problems in isolation from chemical problems present with ZnS in an aggressive plasma. The aspects of substrate preparation, adhesion and stress relief are discussed. The optical and structural properties of diamondcoated samples are presented.

Surface and bulk absorption in germanium at 10.6 μm

The surface and bulk optical absorption of germanium as a function of resistivity and surface condition has been measured at 10.6 microm using adiabatic laser calorimetry. It was found that with conventional optical polishing {Linde, Al(2)O(3)), up to ~50% of the total absorption in a 1-cm thick sample could be attributed to the surfaces. Subsequent polishing with colloidal silica (Syton) reduced this surface contribution to <10% of the total. The remaining surface absorption was not due to volatile impurities which could be removed by evacuation. The optimum resistivity range for germanium having the lowest bulk absorption coefficients at room temperature was found to be 10-40 ?-cm. It was also found that surface absorptions from germanium samples prepared by single-point diamond machining could be lower than those obtained after polishing with colloidal silica, although in many cases they were considerably higher.

Biased enhanced nucleation of diamond on metals: an OES and electrical investigation

Abstract Biased enhanced nucleation (BEN) on non-diamond substrates is poorly understood and displays problems of reproducibility. In this work, optical emission spectroscopy (OES) and electrical measurements have been used to study the BEN process. OES reveal changes in the chemical species within the plasma that occur as a result of the applied bias to a tungsten substrate. As the bias is applied the atomic hydrogen intensity and the ratio of C 2 to CH species detected changes considerably. Both effects appear to be greatest near the substrate surface. The results are discussed in terms of possible origins for the bias enhanced nucleation process. Electrical measurements indicate that reactor pressure may be responsible in part for the reproducibility problems often associated with the BEN technique.

Optical characterization of textured microwave CVD diamond

Abstract Diamond which exhibits strong preferred orientation in 〈111〉, 〈110〉 and 〈100〉 directions was grown in thicknesses of up to 200 μm by microwave chemical vapour deposition in an Astex reactor. Characterization by X-ray diffraction, scanning electron microscopy, Raman spectroscopy and IR transmission on both free-standing diamond and diamond on silicon substrates was performed. It was found that there is a variation in optical transmission with both the preferred orientation of the diamond and the layer thickness. The lowest CH absorptions were obtained for the 〈110〉 material followed by the 〈100〉 and then the 〈111〉 diamond. The defect-induced single-phonon absorption was highest in the 〈111〉 textured material but no significant absorption difference between the 〈100〉 and 〈110〉 textures was found. By the addition of suitable amounts of oxygen, both the CH-related and the single-phonon absorption could be reduced to undetectable levels. A decrease in CH absorption with layer thickness may be related to the poorer quality and microstructure of the diamond close to the growth interface.

A microbeam analytical characterization of diamond films

Abstract Diamond films prepared in an ASTeX plasma deposition system and the Heriot-Watt UHV compatible microwave deposition system have been characterized by a range of techniques including electron microscopy, electron diffraction, electron energy loss spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary ion mass spectrometry. XPS studies have concentrated on a comparison of the valence band structure, the X-ray excited Auger peaks, and the characteristic loss region of the carbon l s photoelectron peak. AES work has involved a study of the structure of the C KVV Auger peak. Microtwinning and grain structure in the diamond films has been studied by TEM. In EELS the fine structure at the carbon K-edge has been studied to give information on the form of carbon bonding. Reflection electron diffraction has been used to determine the interplanar spacings of the films. The interface formed between the diamond and the silicon substrate has been studied by electron beam induced conductivity (EBIC) in the SEM. The film morphology has also been studied in the SEM.