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4 APPLICATIONS New methods for producing superhard materials have enormously enhanced their potential applications in both old and new areas of technology. These applications generally will depend on a combination of properties. For example, the use of SiC and diamond as new semiconducting electronic materials will depend both on their electronic properties ant! on their thermal properties. Application of diamond as a substrate for electronic packaging takes advantage of a very high thermal conductivity for large heat dissipation, very high electrical resistivity for excellent electrical insulation, and low permeability for environmental protection of devices. Tables 4-1, 4-2, and 4-3 list the combinations of properties that make superhard materials desirable for ~ wide range of applications. Six categories of materials have been chosen: single- crystal diamond, polycrystalline diamond composite, polycrystalline diamond film, diamond-like materials, SiC, and cubic BN. Specific applications are discussed in more detail in the following sections. TABLE 4-1 Mechanical and Thermal Applications of Superhard Materials Application Property Utilized Material Bearings, journals ABCDEF 23456 Barrier coatings BC F 3456 Heat sinks Ft 123456 Cutting tools AB DEE, 123456 Surgical blades' A D 1 3456 microtomes Knives, AB DE 1 3456 cutting blades Abrasives A F 23 56 Medical implants ABC E 34 Wire drawing dies AB DEF 123 5 Legend A- Hardness B - Chemical stability C - Permeability D - Young's modulus E - Friction coefficient F - Thermal conductivity 1 - Diamond, single-crystal 2 - Diamond, polycrystalline composite 3 - Diamond, polycrystalline film 4 - Diamond-like materials 5 - Silicon carbide 6 - Cubic boron nitride 73
74 TABLE 4-2 Electronic Applications of Superhard Materials Application Property Utilized Material Transistors High-performance F LM 1 56 High-temperature BC F LMN 1 56 Radiation-hard F J LM 1 56 Hig}~-power F LMN 1 56 Negative electron affinity cathode J L 1 3 Electrical insulators B JK M 1 3456 Substrates, heat sinks F K M 1 3 X-ray make ABCD G 134 Direct-write microelectronics AB 4 Magnetic diske, heads AB E 1 4 Hith-`roltage switches F K M 1 S6 Thermistors F 3 13 56 Packaging A C F K M 1 34 Legend A- Hardness B - Chemical stability C - Permeability D - Young's modulus E - Friction coefficient F - Thermal conductivity G - X-ray tranemimion H - Infrared trar~m~ion I - Visible tranerniesion J - liltra~riolet transmission, large band-gap K - Electrically insulating L - Semiconductivity M - Dielectric strength N - Carrier velocity\mobility 1- Diamond, single-crystal 2 - Diamond, polycrysta}line composite - Diamond, polycrystalline film - Diamond-like matenal 5 - Silicon carbide 6 - Cubic boron nitride Many of the potential applications of superhard materials are currently limited by conditions during synthesis and by the properties of the resultant product. In the case of diamond films, limiting factors are high substrate temperatures during deposition; large surface roughnesses that are undesirable for many mechanical and optical applications; poor adherence to substrate materials; anti stresses in the films that can lead to delamination and substrate fracture. Other applications require single crystals or epitaxial films. Single-crystal diamond films have been grown homoepitaxially on diamond; however, epitaxial growth on other materials, which would be highly desirable, has not yet been confirmed. For electronics, one must be able to dope with electrically active species to produce both e-type and p-type conductivity. III the case of diamond, there is a problem of how to incorporate substitutional e-type dopants. Finally, CVD deposition of polycrystalline bulk material, as opposed to thin film material, requires high deposition rates. Although high deposition rates have been reported in the literature, uniform deposition of high-quality material over large areas has not yet been achieved.
75 TABLE 4-S Optical and Optoelectronic Applications of Superhard Materials Application Property Utilized Material Windows, lenses, mirrors ABCD P. HIJ 1 X-ray windows ABCD FG 13456 Mirror substrates ABC F 1 Heat sinks F K M 1 ~ 56 Optical emitters, lasers, LEDs F J L Optically activated switches F J LMN Ultraviolet detectors J L Optical waYeguide C HI O 1 56 1 56 1 6 1 ~ Legend A- Hardness B - Chemical stability C - Low permeability D - Young modulus E - Friction coefficient F - Thennal conductivity G - X-ray transmission H - Infrared tranamise~on I - Visible transmission J - Ultraviolet tranam~sion, large band-gap. K - Electrically insulating L- Semiconductivity M - Dielectric strength N - Carrier ~relocity\mobility O - Refractive index MECHANICAL APPLICATIONS Bearings and Journals: Coatings and Monolithic 1 - Diamond, single-crystal 2 - Diamond, polycrystall~ne composite 3 - Diamond, polycrystalline film 4 - Diamond-like material 5 - Silicon carbide 6 - Cubic boron nitride Single-crystal diamonds have been used as bearings and wear-resistant surfaces (as for phonograph needles) and more prominently for wire-drawing dies. This last use has been taken over by polycrystalline sintered diamond, which eliminates the problems of anisotropic wear (due to differential hardness) and weakness due to cleavage of single crystals. These compacts are available from the principal diamond manufacturers as layers up to several millimeters in thickness and 50 mm in diameter on cemented carbide or as cylindrical slugs of diamond at least 14 mm in diameter by 14 mm high. They are used as the "fillings in the hole of a doughnut of cemented tungsten carbide. These slugs are used primarily for wire-drawing dies, which are precision products with controlled grain size that improve the surface finish of wire. Tungsten carbide with a coated layer of sintered diamond is a proven product for well-drilling and cutting tools. Diamond-carbide composites are also used for bearing surfaces, but there is great opportunity for further innovative applications. In their manufactured form, these compacts contain several percent of a metal phase as a result of the liquid sintering process. This phase imposes a maximum operating temperature of about 750°C. Above this temperature, catastrophic cracking failure occurs because of thermal expansion mismatch and incipient graphitization. It is possible to remove almost all of the metal phase by acid leaching, thus eliminating this cracking failure mechanism. However, the temperature limitations of graphitization and oxidation of diamond are still present. Cubic BN
76 can be used at higher temperatures than diamond. Surface fluorination is being used to enhance the oxidation resistance of diamond (Margrave, et al., 1988~. Some of the advantages seen for CAD diamond layers in bearing and wear-surface applications are higher thermal stability, which will aid both in use and in brazing to substrates; essentially theoretical density (no metal phase, no porosity from metal removal); ability to make intricate shapes; and potential of making smooth, mirror-like surfaces without polishing. The high thermal conductivity of some of the superhard materials (diamond, cubic BN, SiC) can decrease wear significantly by rapid transfer of heat from hot spots caused by localized frictional heating. Diamond and diamond-like materials both have shown small coefficients of friction (<0.05), which also acts to minimize frictional wear (Koskinen et al., 1985; Miyake et al., 1985). In the case of diamond-like materials, the coefficients of friction depend on the relative humidity (RH). Up to about 1 percent RH, the coefficients are very low (0.01-0.02), however, above 1 percent RH the coefficients increase rapidly, reaching 0.2 at 100 percent RH (Tsai and Bogy, 1987~. The coefficient of friction of diamond is less sensitive to moisture. In the case of diamond, adsorbed hydrogen appears necessary to maintain the low coefficient of friction. However, in a vacuum environment or at high temperatures, this hydrogen layer will desorb, resulting in a significantly higher coefficient of friction. Although diamond-like films are very smooth, CVD diamond usually has a rough surface that would act to abrade an opposing Dart. Denosition of smooth CVD diamond films would solve this problem. fine nonreact~v~ty ot supernara materials In corrosive environments ana al high temperatures makes possible the fabrication of bearings and journals that can operate in adverse environments such as those found in nuclear reactors. _ ~~ , . .. . - . ~ , . . . · · . . The ability to deposit diamond, DLC, and cubic BN in thin film form should allow for coating large surface areas for abrasion and wear resistance. Thus, for example, it may be possible to produce diamond-coated ways, spindle bearings, and shafts on milling machines and lathes. The decrease in wear of such components would result in equipment that maintains small machining tolerances over significantly greater periods of time than at present. Barrier Coatings Nonreactivity in corrosive environments, at high temperature, and in the presence of ionizing radiation may make it possible to use some superhard materials as protective barrier coatings in adverse environments. Their low permeability would prevent diffusion of reactive species to the substrate material, thus preventing substrate degradation. High thermal conductivities would diffuse hot spots, preventing localized high temperatures from reaching the substrate. Cutting Tools: Drills' Bits. Cutters. Saw Blades Precision turning tools made of polycrystalline diamond are still of limited value because the grain size is larger than the desired surface finish in most cases. This is less of a problem in direct-converted polycrystalline materials (i.e., graphite converted directly to diamond without solvent catalysts, or hexagonal BN converted to cubic BN). CVD diamond and cubic BN are very attractive for this application because of the possibility of having very-fine-grain material or single crystals deposited and bonded directly onto the tool material.
77 Surgical Blades and Microtomes Single-crystal diamond is already used for microtome blades and surgical knives. The sharpness of the blade allows cutting of biological tissue without significantly depressing the tissue (minimum springback), anti the hydrophobic surface of diamond prevents sticking and dragging. The production of large synthesized single crystals was at first aimed primarily at this market and the turning too! market. The limitations here are primarily associated with the difficulty of grinding and polishing uniform edges. It is expected that eventually the CVD process could be used to produce a uniform edge directly, without a polishing procedure, and thereby solve this problem. Knives and Cutting Blades There are applications for cliamond cutting blades in a wide variety of industries. For example, the orange juice industry requires cutting blades that maintain their sharpness for an extended length of time. Dull blades waste the juice in the oranges. At the present time, these cutting blades, which are made of a hard sta~niess-stee] alloy, require sharpening every 30 days. If the blades could last for a full cutting season of 9 months without sharpening, considerable savings would result. Coating the cutting blades with diamond or DLC would appear to be one way of extending the time between sharpenings. Abrasives Diamond grain and cubic BN abrasive grain were the first synthesized superhard materials products. Sintered polycrystalline materials followed. Manufacturers in about 14 countries make ~ood-aualitY abrasive grain that ends up mounted in various media for cutting. grinding. and sawing applicatlonse Tram sizes range Tom submlcrometer scale to almost 1 mm. l He larger sizes are used primarily for rock cutting and shaping. Synthesized abrasive grain can be provided reproducibly with controlled properties--size, shape, toughness. It is for this reason that about 90 percent of industrial diamond is synthesized and 10 percent is natural. The potential of CVD diamond for abrasive grain is now being evaluated by various manufacturers. It will probably find a unique niche in the market by virtue of its purity, shape (i.e., flakes and sheets are possible), and polycrystalline nature. Its defect structures may give CVD diamond different and perhaps better abrasive properties. Medical Implants Ion beam-deposited DLC coatings have been studied for their use in biological implant systems (Aisenberg and Chabot, 1974~. The coatings produced were found to have an extremely smooth surface, high surface tension, and slight electronegativity (Adamson, 1982~. In addition, the samples were found to have a low gas permeability and high adhesion to the substrates tested. Results of vena cave, renal embolus, and ex viva tests indicate a high degree of thromboresistance for these materials. These results combined with the durability of the materials suggest that they could be useful as coatings for medical implants, including artificial joints. One would expect that diamond coatings would perform in a similar manner. The high surface energy of diamond, which causes it to be hydrophobic, allows it to resist bacterial and viral attachment, since surface energy relates to surface tension (Adamson, 1982~.
78 Additional Technological Impacts A wide variety of technologies would benefit from the use of superhard materials. These include bearings for cruise missiles and helicopter rotors; diamond-tipped bonding tools for electronic leads; oil-drilling bits; gourmet knives; thermal and rocket engines, loudspeakers with extended frequency response (first commercial application of DLC films); coatings for dissipating heat and ionizing radiation in fission and fusion reactors; diamond-coated storage containers, such as beakers, for holding highly reactive, corrosive chemicals; gears; seals. ELECTRONICS Superhard semiconductor materials are expected to have an enormous technological impact on electronics, especially in the areas of high-power, high-performance transistors (Bazhenov et al., 1985; Gels et al., 1987; Prins, 1982; Setaka, 1986, Moazed et al., 1988~. Two recognized figures of merit exist for evaluating semiconductor materials (Johnson, 1965; Keyes, 1972~. The first of these (chronologically), formulated by Johnson, applies to the evaluation of semiconductors for performance as high-power microwave and millimeter-wave amplifier materials. Its principal parameters are the saturated charge carrier velocity (he., the velocity at which charge carriers move through the semiconductor at very high electric fields) and the dielectric breakdown strength. Table 4-4 lists the relative ranking of Johnson figures of merit of important semiconductor materials. (Parameters necessary to estimate the Johnson figure of merit for BN have not been available.) Obviously, the high-temperature semiconductors exhibit great potential for improving the performance of microwave and millimeter-wave power amplifiers. TABLE 4-4 Relative Figures of Merit for Electronics Ratio to Silicon . . Material Johnson: Keyes2 si , O GaAs 6.9 0.5 InP 16 0.6 c'-SiC 694 ,B-SiC 1 1 38 5.8 Diamond 8206 ~ 32 . 1High-power, high-frequency microwave amplifiers. 2Dense integrated circuit applications. Keyes figure of merit: aT VSAT K
79 Johnson figure of merit: EB VSAT where {T = -T thermal conductivity V SAT = saturation velocity EB = breakdown voltage K = dimensionless quantity Keyes formulated a figure of merit for choosing semiconductors for dense integrated circuit applications. Principal parameters used in this formula are the dielectric constant saturated velocity, and thermal conductivity. Table 4-4 also lists the relative ranking of Keyes figures of merit for the same set of semiconductors; again, the parameters for BN are not certain enough for use at this time. Here also one sees clearly the superiority of §-SiC and diamond. To have successful electronic device applications, doping of diamond is essential. Recent results indicate that p-type doping is easily achieved with boron by incorporation using CVD continued improvements are seen in both CVD and in ion implantation techniques. At the writing of this work, progress is also seen in e-type doping of diamond, both by ion implantation and CVD techniques. Ion implantation utilizing low-temperature carbon damage with overlapping boron implantation followed by rapid thermal annealing produced successful boron doping comparable to CVD methods. Preliminary work on e-type doping shows encouraging results. Theoretical studies predict that lithium, sodium, phosphorus, and antimony are excellent candidates for n- type doping. Other areas of major potential impact of superhard materials are electrical insulators, high- temperature transistors, negative electron affinity cathodes (Himpse} et al., 1979), electronic heat sinks, advanced projection photolithography, clirect-write microelectronics, transistors resistant to ionizing radiation, and coatings for magnetic rlisks and magnetic read-write heads for protection from head crashes. High-Power Transistors - The output power that a transistor can deliver to a load resistance is proportional to the square of the voltage across the load. Superhard materials such as SiC EN, and/or diamond exhibit dielectric breakdown strengths of 4 x 106, 1 x 107, and 2 x 107,volts/cm respectively. This compares to silicon breakdown at 5 x 105 volts/cm. The tenfold increase in breakdown strengths of the superhard materials translates into potential power output increases of 100-fold over silicon devices. This advantage is of particular importance in phased array antenna modules--especially for air search and space search surveillance radars. Other advantages accrue in replacements for vacuum tubes. Cathode wear-out is the primary failure mechanism for microwave and millimeter-wave vacuum tubes (traveling-wave tubes) in satellites.
80 High-Performance (Rapid-Response! Transistors Several factors affect transistor speed. Among them are dielectric constant, charge carrier mobility, and charge carrier velocity. The dielectric constant determines the parasitic capacitance of the transmission lines on integrated circuits. Phase distortion and propagation speed are adversely affected by a large parasitic capacitance. The dielectric constants for diamond and SiC are 5.5 and 9.0, respectively, compared to 12 and 13 for silicon and germanium, respectively. Speed/frequency performance is inversely related to the dielectric constant, therefore, the superhard materials provide advantages in applications involving supercomputers and solid-state power amplifiers in the microwave and millimeter-wave spectrum. Modern transistors require lithographic resolutions of less than 1.0 micrometer and are characterized by charge carrier channel regions of similar dimensions. These very short channel lengths lead to electric field strengths of 50,000 volts/cm in logic devices and even greater fields in analog devices. Since the superhard materials have lower atomic mass than more conventional semiconductors, the energies of the optical phonons that scatter the charge carriers are much higher. The velocities at which the charge carriers (i.e., electrons and holes) are scattered by optical phonons is thus much higher, and charge carrier transit time across the transistor channel is therefore shorter than in semiconductors characterized by higher atomic mass. Transit time gain factors of 2 to 3 over silicon and GaAs are possible. This translates directly into a proportionately higher frequency response. Although charge carrier mobilities exhibited by diamond and SiC are generally lower than those of conventional semiconductors (an exception is the hole mobility of diamond, which is 3 times the hole mobility of silicon), this limitation can be circumvented by using cascade- structured devices, wherein the charge carrier velocity entering the second stage of the device is near saturated velocity. In this manner the average velocity of electrons in diamond and SiC can be much higher than that of even GaAs. A final speed and frequency advantage of diamond and SiC over III-V semiconductor devices arises because the superhard semiconductors do not exhibit a region of negative differential conductivity. As such, the charge carrier velocity remains high, even at high electric field strengths. This equates to higher-power microwave and millimeter-wave amplifiers. Hi~h-Temperature Transistors The large forbidden band-gaps of diamond, SiC, and cubic BN enable them to continue to act as semiconductors at extremely high temperatures without experiencing charge carrier degeneration. Temperatures as high as 6500C have been demonstrated for -Sib; BN and diamond should perform at even higher temperatures. Practical upper operating temperatures will be limited primarily by degradation in the metallization used for ohmic and Schottky contacts to the semiconductors. High-temperature transistors will be particularly advantageous in spacecraft. In unmanned spacecraft, the systems required to cool conventional silicon devices to 125 °C could be largely eliminated if operating temperatures of 325°Cwere permissible. These cooling systems can occupy up to 60 percent of the volume of an unmanned satellite. Thus the use of semiconductors such as SiC, cubic BN, and/or diamond could significantly reduce the system volume, weight, and cost. However, the advantages, compared to silicon and GaAs, may be offset because the mobilities of the negative free carriers (holes) appear to decrease more rapidly with increasing temperature than those of silicon and GaAs.
81 Radiation-Hard Transistors The binding energies between the atoms of superhard crystals are much stronger than those of the more conventional semiconductors. Therefore, their vulnerability to heavy particle (ion) bombardment is less than that of conventional semiconductors. Furthermore, because of their higher band-gaps, fewer electron-hole pairs are generated per impinging photon; again this translates into greater immunity to alpha, beta, and gamma radiation. Definitive tests on diamond and BN devices have not yet been conducted because of the embryonic state of the art. Preliminary tests on synthetic epitaxial and natural diamond indicate a factor of 100 improvement in the damage resistance over that of silicon to 1.5 meV electrons. Initial Japanese tests on ,B- SiC devices have shown them to be immune to a radiation dose of 107 red equivalent in silicon, radiation sufficient to damage most silicon devices. If fused silica is used as the insulator for SiC devices, the fused silica and not the SiC will be the limiting factor in radiation immunity. Fortuitously, AIN is lattice-matched to SiC and may be useful as a heteroepitaxial insulator, enabling SiC to operate in a potential well of the single-crystalline A1N insulator. If so, it may be possible to fabricate insulated-gate field-effect transistors that do not have the limitations of present SiO2-based silicon devices. Similarly, diamond devices may be made to operate in a potential well created by lattice-matched cubic BN insulators. A significant amount of research will be required before these aspects of inherent radiation hardness can be confirmed. The combination of radiation hardness and high-temperature capability of superhard materials may make electronics practical within or near nuclear reactors. High-Voltage Electronics High-voltage electronics made from superhard semiconductor materials would have greater reliability than vacuum tubes, the components most likely to fail in high-voltage circuits. This would be extremely important for phased array antenna modules and satellites. Megavolt switches would be reliable because superhard materials have high breakdown voltages. This would facilitate implementing high-voltage dc transmission of electricity. Reliable Electronics The predominant failure mechanism in power semiconductors is the migration (diffusion) of dopant impurities. The superhard semiconductors have a decided advantage in this respect. The diffusivity of dopant impurities in the superhard materials at temperatures well above the melting point of silicon is equivalent to the diffusivity of dopants in silicon at 125 C. At temperatures well above 400 C, impurity diffusion in the superhard semiconductors is negligible. The predominant failure mechanism will be solely that of the metallization. The use of metal silicides and metal carbides is expected to significantly improve resistance to even this degradation mechanism. However, low diffusivity may be a problem when trying to obtain uniform semiconductor doping. Neeative Electron Affinity Cathodes Diamond is believed to have a negative electron affinity (Himpsel et al., 1979~. This should make possible efficient electron emitters for a variety of applications, such as electron guns with cold cathodes and micrometer-sized vacuum tubes that would have power and frequency performance far superior to that of conventional semiconductor devices.
Electrical Insulators 82 Superhard semiconductor materials generally possess large forbidden band-gaps and therefore are good electrical insulators. Natural diamond, for instance, can exhibit dark resistivities greater than 102° ohm-cm resistance. In synthesized diamond, resistivities vary between 10 and 10~° ohm-cm, depending on the growth process used. The insulating properties of semiconducting heterojunctions must, however, be considered on a case-by-case basis. Depending on how the band-gaps align, the material with the wider band-gap may not necessarily be an insulator for both electrons and holes. The view of semiconductor-insulator interfaces must be modified to a concept of assessing materials in potential wells. Diamond-like materials can have resistiYities exceeding 10~2 ohm-cm. Heat Conductors Most of the superhard materials useful for electronic applications also have high thermal conductivities. For example, natural diamond has a value of 20 Wcm~i K-' at room temperature (Ono et al., 1986~. Since natural diamond contains both carbon 12 and carbon 13 isotopes, its thermal conductivity is not optimized. Isotopically pure carbon 12 diamond should have a thermal conductivity that is greater by a factor of 2, but experimental verification of this prediction is needed. BN, SiC, and AIN are also good thermal conductors and are of use in a variety of applications. These materials are most useful in single-crystal form, heteroepitaxially bonded to the material to which they are to be used as heat sinks. However, heteroepitaxy is believed to be limited to lattice-matched materials with similar chemical bonding. BN is only lattice-matched to diamond, and SiC is only lattice-matched to AIN and GaN. Most applications will require other forms of bonding, such as soldering. Unlike copper, the superhard heat sinks are generally electrically insulating in nature and can be used for removing heat without the complicating requirement of an additional electrically insulating layer. The high heat-removal capability of these materials should permit the fabrication of integrated circuits with significantly increased component densities. X-RaY Masks for Advance Projection Photolithography X-ray wavelength masks offer the possibility of achieving significantly higher-density electronics. These masks must be thin to prevent absorption of the x-ray radiation and yet must be optically transparent for visual observation. Large areas of thin unsupported diamond films will be required. Direct-Write Microelectronics Coating of electronic chips during processing with diamond-like coatings can be used to pattern the electronic circuit directly--for example, by electron beam etching. Magnetic Disks and Read-Wr~te Heads Magnetic disks and read-write heads are susceptible to damage when the head comes into contact with the magnetic medium. The result can be destruction of the disk, the head, and
83 valuable stored data. This problem is becoming even greater as attempts are made to increase the density of data on the storage medium, requiring smaller head-to-disk gaps. The disk and head must be protected with a wear-resistant, low-friction coating. Much of the earlier discussion concerned with friction anti wear in bearings and journals is applicable here. Coating of disks and heads with DLC has been the subject of extensive study, and a considerable payoff in saved data may be realized (Tsai and Bogy, 1987~. Additional Technological Impacts Additional advantages of superhard electronic materials include in situ electronic sensing and signal processing in chemical reactors, automobile engines, engines in heavy industrial equipment, rocket engines, nuclear reactors, etc.; low-cost microwave ovens; higher-speed supercomputers; nuclear radiation dosimeters and detectors with great potential for use in radiation biology and medicine; and thermistors. OPTICS AND OPTOELECTRONICS The desirability of superhard materials as optical materials stems from their unique properties. Their advantages apply both to monolithic optics and to optical coatings. Their primary advantages lie in their hardness and chemical inertness, which would make optical components fabricated from these materials resistant to adverse conditions such as abrasive environments caused by dust and rain, chemically corrosive environments, reactive environments as in plasmas, and high-temperature environments. In addition, particular materials would have other advantages. The wide band gaps of diamond and cubic BN make them candidates for ultraviolet optics, and their low mass numbers make them candidates for soft x-ray windows. High thermal conductivity, high Lounges modulus, low thermal expansion coefficient, and high fracture toughness make diamond an important candidate material for optical components in high-photon-flux environments, such as in free-electron lasers, because of its resistance to thermal fracture and its ability to dissipate heat rapidly. Table 4-5 shows the figures of merit for thermal stress resistance of diamond relative to those of several important optical materials. Diamond is clearly superior to the other materials, from both a practical and a theoretical point of view. Several major problems must be overcome before diamond optics in the near-infrared, visible, and ultraviolet wit} be widely used. In the case of single-crystal diamond, only limited sizes are available, perhaps up to 2 cm in diameter. CVD diamond could be used in monolithic optics (such as lenses, winclows, and domes), provided deposition rates were sufficiently high. In the case of CVD diamond, however, the limitations are optical scatter arising from polycrystaIline morphology and surface roughness and absorption resulting from graphitic or diamond-like species, lattice defects, and other impurities within the diamond phase. Crystalline diamond and diamond-like materials have many potential uses as coatings for infrared optics. In principle, diamond shows no first-order absorption in the infrared; the absorption observed is clue to second-order processes, defects, and impurities. Diamond coatings are potentially applicable for use from the nonvacuum ultraviolet through the infrared, except for a region of absorption between 2.5 and 6 ~m, but optical scatter limits such use in currently produced CVD diamond. However, at wavelengths beyond approximately 10 ~m, transmission measurements suggest that this material may have adequate transmissivity to be useful in the longwave infrared region.
84 TABLE 4-5 Figures of Merit for Thermal Stress Resistance . .. . . Material RT Source Silicate glass 2.3 x 102 a Sapphire 3.4 x 103 a Fused silica 4.3 x 103 a Diamond (300 K) 9.4 x 105 b ( 80 K) 1.3 x 107 b (300 K) 3.8 x 108 c ( 80 K) 5.4 x 109 c a. From Krupke et al. (1986), in which a flaw size of 50 I'm was assumed. b. Based on parameters in Field (1979) and assuming a flaw size of 50 am. c. Theoretical value based on parameters in Field (1979). RT = Of )c ( l ~ ) cxE where RT = figure of merit of ~ fracture strength arc = thermal conductivity c' = Poisson's ratio = linear thermal expansion coefficient E = Young's modulus The use of CVD diamond as an optical film material has the limitation of requiring high substrate temperatures (greater than 600 °C)during deposition, which restricts the number of substrate materials that can be used. Adhesion to the substrate is another problem, and, when the diamond does adhere, differential contraction during cooling can lead to delamination and crazing. Diamond-like films, on the other hand, can be deposited at much lower temperatures (approximately 150 °C),but they too exhibit adhesion problems and may also contain large internal stresses that can cause delamination of thick coatings from substrates. Hydrogen-free DLC coatings have much less residual stress than the DICK type, however. Some of the early adherence problems have been addressed with good results. For example, the adherence of diamond-like films to infrared-transmitting substrates such as ZnS and ZnSe is improved dramatically by depositing an amorphous germanium-carbon layer between the substrate and the overlying diamond-like coating (Lettington et. al., 1987; Wort and Lewis, 1987~.
85 This combination of hard coatings can impart good wear erosion and abrasion resistance to ZnS and ZnSe and can be adjusted in thickness to provide an antireflection coating as well. In addition to their potential use as optical coatings and monolithic optics, superhard materials have potential uses as mirror substrates, optical and optoelectronic heat sinks, laser sources, light-emitting ciiocies (visible and ultraviolet), high-speed optically activated electronic switches, ultraviolet detectors, optical waveguides, and fiber optic coatings. Monolithic Windows and Lenses The ability to deposit high-optical-quality CVD diamond at high deposition rates will make it possible to produce winclows and lenses for the infrared, visible, and ultraviolet that are durable in extreme environments. In addition to the environmental advantages of diamond, its high refractive index makes it an attractive lens material because lenses with high refractive power can be made significantly thinner and hence lighter. Furthermore, simple antireflective coatings can be easily made for diamond optics, especially for the ultraviolet, because the antireflection coalition is more easily satisfied than for other substrate materials. For example, a single-layer antirefiection coating, to work efficiently, must have a refractive index close to the square root of the substrate refractive index, a condition difficult to achieve for substrate refractive indices less than 1.9. The refractive index of diamond at 0.589 Em is 2.418; a film with an index of 1.555 would make an almost perfect antirefiection coating, a value easily obtainable with optical glasses. In addition, diamond and DLC make good infrared antireflection coatings for substrate materials such as GaAs, silicon, and germanium. Single-crystal diamond has particular advantages as a high-power laser window or output coupler, such as is required in a free-electron laser. Because of its superior thermal shock resistance and its large heat-dissipation capability, it may make possible very compact free- electron lasers. With diamond windows the areal power density could be higher, resulting in greater total power for a given beam size. If the windows are cooled to liquid-nitrogen temperatures, the heat-dissipating capability of diamond windows would be even greater. Thus, an effort to produce large, high-optical-quality single-crystal diamond either by high-pressure techniques or by CVD processes would be extremely desirable. X-Rav Windows , DLHC, and BN, these materials exhibit low x-ray absorption, making them excellent x-ray windows. Because of its high strength, diamond can be made very thin, thus lowering its absorptivity even more. In the soft x-ray region, diamond would be expected to replace beryllium. Commercialization of diamond for energy-dispersive anaylsis windows for energy-dispersive x-ray analysis is expected shortly. Because of the low atomic numbers of the elements in diamond, DLC Mirror Substrates High thermal conductivity and high strength make diamond an excellent substrate material for mirrors intended for high-average-power laser radiation. The high strength would allow for thinner substrates and hence lighter mirrors. This will be practical when large-diameter, thick deposits can be made at reasonable deposition rates.
86 Heat Conductors Diamond, SiC, cubic BN, and AIN can act as heat-conducting substrate materials for optoelectronic applications such as laser diodes, where significant heat may be developed. In this case the constraints are similar to the electronic ones discussed earlier. The use of superhard materials as mirror substrates, as discussed above, takes advantage of their large thermal conducting capabilities. Optical Emitters. Lasers. and Litht-Em~tting Diodes SiC and cubic BN, because of their large direct band-gaps, emit optical radiation at significantly shorter wavelengths than conventional III-V materials. BN has recently demonstrated emission in the ultraviolet. Furthermore, it is expected that light-emitting diodes can be made from these materials. In diamond, direct laser action is expected to be more difficult because it is an indirect band-gap material. However, laser action has been exhibited by color centers in diamond (Rand and DeShazer, 1985a, by in the visible region; thus, diamond can act as a laser host material. Calculations indicate that, by applying a large uniaxial strain to diamond, it can be converted to a direct band-gap semiconductor. Whether such a large uniaxial strain can be produced and maintained in practical devices is highly problematical. Perhaps it can be accomplished by making diamond one component of a strained-layer superlattice. Other possibilities for laser action in diamond are intra-atomic energy transitions (2d and 4f) in impurity ions within the diamond or within a heterojunction pen junction. Ultraviolet lasers made of superhard materials offer greater storage density capability on compact disks because the laser light could be focused to smaller spot sizes. Optically Activated Electronic Switches Recent experiments have shown that diamond can act as an optically activated electronic switch (Bharadwa; et al., 1983; Young et al., 1983; Glinskietal., 1984; Huo etal.,1986~. Because of its high breakdown voltage, diamond can withstand very high voltages without electrical breakdown. Illumination by intense excimer laser radiation leads to switching times on a nanosecond time scale and smaller. Ultraviolet Solar-Blind Detectors In the region between 180 nm and 300 nm, the ozone layer reduces the reflectance of the earth over 3 orders of magnitude below that of the visible and near-infrared. In the region of 140 to 180 nm, the reflectance is further reduced by an additional 200-fold. Below 140 nm, aural spectra dominate. For exoatmospheric "targets" a truly solar-blind region is the window at 140 to 180 nm. Although the band-gap of cubic BN is not now known with certainty, it is expected to lie within this window. Since BN has now been synthesized in its cubic form, the exciting possibility exists that BN can be used not only as a solar-blind detector but also as a coherent (laser) illuminator as well. Diamond should be blind to radiation at wavelengths longer than 225 nm and thus can be used to capitalize on the more than 3 orders of magnitude ozone-created reduction in the solar reflection of the earth. SiC has too low a band-gap to be useful in solar- blind applications. , - . ..
87 Optical Waveguides The high refractive index of diamond makes it a candidate optical waveguide material. Because it is a centrosymmetric material, it will not exhibit an electro-optic effect; thus, it might be useful either as a passive waveguide material or as an acousto-optic material. Furthermore, the refractive index of diamond is between the refractive index of -Gads and related IIT-V compounds used as optical sources and that of LiNbO3, a modulator material. Thus, diamond can act as a transition waveguide between components made of these two classes of materials. Fiber Optic Coating Fiber optics are subject to abrasion and chemical degradation from moisture. The strength of silica-based fibers is severely lowered by water vapor. Hard, impervious coating materials are needed to protect these fibers. Both diamond and diamond-like carbon are hard, providing abrasion resistance. In addition, because they are dense and chemically impervious, they can be used to hermetically seal a fiber from the environment. Fiber optics coated with DLC are effectively sealed against water-vapor penetration. This will be especially important for the new heavy-metal halide fibers that exhibit low absorption in the infrared but are particularly susceptible to environmental degradation, especially moisture. Optical communications systems, including military systems, rely heavily on fiber optics. Optical Coatings High-Power Laser Mirrors Laser mirrors are subject to degradation from a variety of sources: They are subject to intense optical radiation, which causes a form of dielectric breakdown owing to the high optical electric fields present; they are subject to thermal shock and degradation as a result of heating by the intense optical radiation; they are subject to attack by reactive species such as fluorine in excimer lasers; they are subject to plasma erosion in lasers with gas discharges; and they are subject to environmental degradation from dust and moisture when they act as the output coupler to the external environment. Furthermore, many lasers operate in the ultraviolet. where there is a , lack of hard coating materials possessing high refractive indices. Because of the high refractive index of diamond and its resistance to environmental attack, the development of low-optical- loss diamond coatings in single layers and in multilayers with other lower index materials such as SiO2 would considerably ameliorate these problems. The high refractive index of diamond should make possible antireflecting and high-reflecting mirrors with fewer layers and with greater survivability than current mirrors. Protective Coatings Many optical components are subjected to extreme environmental factors that cause component degradation. Much of this degradation occurs in environments of critical importance to the nation's defense. Aircraft fly through dust and rain, which abrade and erode aircraft canopies; tanks and personnel operate in dusty and moist environments that degrade viewing ports, binoculars, periscopes, rangefinders, target designators, and goggles, radomes and IR- domes are subjected to dust and rain impact and abrasion and may be heated to elevated temperature because of atmospheric friction.
88 Heating of radomes in re-entry vehicles can be a problem because the high temperatures generated in the atmosphere can cause the radome material to become conducting, which prevents the transmission of microwave energy owing to absorption. In this situation diamond radomes would have four advantages: because of its large band-gap, diamond would remain insulating to elevated temperatures; the low dielectric constant of diamond results in low reflectivity of microwave energy because of a lower impedance mismatch with the air; the high thermal conductivity of diamond would permit generated heat to dissipate over a larger area, thus lowering peak temperatures; and oxidation of the diamond surface would cause production of CO and CO2, with the result that the chemical composition of the radome surface would remain unchanged. From a commercial viewpoint, superhard protective optical coatings would be useful on almost all optical surfaces. Diamond-coated eyeglasses, windshields, and windows could significantly maintain visibility by decreasing the susceptibility to scratching. The lifetimes of optical disks would be significantly extended. Improvement by a factor of 6 in scratch resistance has been obtained for plastic lenses coated with DLC. Antirefiection Coatings Antirefiection coatings of diamond or DLC would have considerable advantages. Silicon and GaAs solar cells coated with diamond would exhibit greater efficiency because of reduction of reflective losses, while at the same time the cells would be resistant to degradation from the environment. DLC coatings have been deposited on silicon solar cells with an increase in cell efficiency of 40 percent and on germanium windows, resulting in excellent transmissivity. The advantage of DLC coatings over diamond is that the refractive index of DLC can be tailored to suit a particular application. Values of refractive index between 1.S and greater than 2.1 are obtainable by varying the deposition conditions. Moreover, DLC coatings show a negligible surface roughness. - Additional Technological Impacts Additional advantages that may arise from the application of superhard materials to optics include environmentally stable interference filters; missile detection sensors; and detectors resistant to ionizing radiation. REFERENCES Adamson, A. W. 1982. Physical Chemistry, Fourth Edition. New York: John Wiley and Sons. Aisenberg, S., and R. W. Chabot. 1974. Ion Beam Deposited Carbon Coatings for Biocompatible Materials. National Institute of Health, Contract No. NIH-NOL-HB-3-2919. Bazhenov, V. K., I. M. Vikulin, and A. G. Gontar. 1985. Synthetic diamonds in electronics (review). Sov. Phys. Semicond., Vol. 19, pp. 829-841. Bharadwaj, P. K., R. F. Code, H. M. van Driel, and E. Walentynowicz. 1983. High voltage optoelectronic switching in diamond. Appl. Phys. Lett., Vol. 43, pp. 207-209.
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