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Diamond has some of the most extreme physical properties of any material, yet its practical use in science or engineering has been limited due its scarcity and expense. With the recent development of techniques for depositing thin films of diamond on a variety of substrate materials, we now have the ability to exploit these superlative properties in many new and exciting applications. In this paper, we shall explain the basic science and technology underlying the chemical vapour deposition of diamond thin films, and show how this is leading to the development of diamond as a 21st century engineering material. Jstor

Page 1 www.semi1source.com , posted January 23, 2007Copyright © Andrzej Badzian, 2007, All rights reserved.THE STATUS OF DIAMOND ELECTRONICSAndrzej BadzianPenn State University, University Park, PA,, USATHE RISE OF DIAMOND ELECTRONICS Typical semiconductor materials are associated with group IV elements of the periodic table and AIII-BVand AII-BVI compounds and their solid solutions. The crystal structures of all these phases relate to the diamond atomic structure, so diamond crystal is the model phase for all these, tetrahedral bonded atomic structures. They appear as cubic or hexagonal phases.The first hint for exploration of diamond as a semiconductor material was noted in 1952, when naturaldiamond crystals with blue color have been found to show a semiconductor type of conductivity. Boron atoms, substitutionally located in diamond crystal lattice, cause the color and are responsible for p-type conductivity. In the same year a patent application was filled at Union Carbide to cover the Chemical Vapor Deposition process for diamond growth from the gas-phase. A hope was associated with the vapor growth for development of electronic quality diamond semiconductor films. Nevertheless, it took almost 36 years of research to grow single crystal epitaxial diamond films and to fabricate rudimentary electronic devices. The controlled growth of diamond films was the necessary condition for testing the feasibility of diamond electronics.Before electronic diamond films became available, the research was focused on bulk natural diamond crystals. In this category, synthetic diamond crystals, grown by High Pressure/High Temperature (HP/HT) process were also explored. This process, disclosed in 1962 by General Electric, enabled doping diamond with boron during growth. The start of diamond electronics research was not easy because scientific community was skeptical about reliable diamond growth from the gas phase. At the beginning of the 1980's Russian and Japanese papers clearly described CVD processes yielding diamond films. The role of Penn State University was to confirm the process and dismiss the doubts. The announcement about this achievement in New York Times in 1986, made CVD process attractive for researches around the world. Penn State CVD data together withbulk semiconductor diamond crystals studies helped to design diamond electronics research program at the Office of Naval Research. The main goal of this wide scope program was making CVD diamond synthesis process practical, applicable to a variety coating applications, and testing feasibility of diamond electronic devices.Electronic study of bulk semiconductor diamond single crystals was focused on the fabrication of proto type devices: point contact diode, high-temperature termistor, light-sensitive switch, optical radiation detector and ultrafast infrared detector. The first transistor on natural crystals was demonstrated in 1982 and on HP/HT crystals in 1987. Also Schottky diodes were fabricated on bulk crystals, they operated at elevated temperatures up to 700oC. At this point of the development of semiconductor diamond technology it has become evident that in order to make progress in active electronic diamond device engineering, significant improvements in diamond thin-film (or planar) technology must be accomplished. Diamond Micrchips cvd hpht information link Page 2 THE FEASIBILITY OF DIAMOND ELECTRONICS The support for ONR research program, among a variety of considerations, came from the calculation of Johnson figure of merit (JFM), which is a product of maximum allowable voltage and cutoff frequency: JFM= EcVs/2where: Ecis critical field (or the breakdown field) in excess of 107V/cmVsis electron saturation velocity equal to 2.7x107 cm/s. JFM is used for comparison of semiconductor materials. Diamond has the highest JFM value, (for p-type diamond 32 times higher than for Silicon) among all semiconductor materials. This indicates diamond potential for power microwave application. The Keyes’ figure of merit (KFM) in turn, considered for digital integrated circuits, combines three material parameters: thermal conductivity, dielectric constant and electron saturation velocity. Here again the value of KFM for diamond was the highest among all semiconductor materials. Based on these numbers, conclusions regarding potential of diamond in electronic device applications were very optimistic. Overall, diamond was considered to be a perfect semiconductor for high temperature, high frequency, and high power device applications. The low sensitivity to high-energy radiation only added to the image of diamond as an ultimate semiconductor material. Following very promising predictions regarding potential of diamond as a semiconductor, major research programs on CVD diamond films were initiated in 1986. The ONR, NSF and other governmental and private organizations support for diamond research lasted for about 12 years. After 1997 the support for thin-film diamond research was substantially reduced in the USA, but continued in other countries. CVD PROCESS The up to date achievements of CVD diamond research can be summarized as follows:* polycrystalline, black, free-standing plates up to 25 cm in diameter and 2 mm thick were grown.* translucent (sometimes almost water-clear) polycrystalline, stress-free plates, up to 10 cm in diameter, are available ($100,000 per plate), as microwave or optical windows.* single crystals, in the shape of cubes, up to 1 cm3, were demonstrated.* single crystal plates up to 4.5 mm across are available from Element Six Ltd.* controlled homo epitaxy was used for successful fabrication electronic devices. DIAMOND ELECTRONIC DEVICES The following device-related studies were carried out as a part of the program:*Ohmic contacts Ohmic contacts have been successfully developed using well known ohmic metals. For example, Ti/Mo/Au layer showed no degradation for 400 hrs at 800oC.* Schottky diodes The Au/Ti contact and Au gate contact to B-doped diamond homoepitaxial (001) films were used to fabricate Schottky diodes as a step leading to fabrication of rudimentary MESFETs. The diode operated atup to 600oC. Considerable rectification was expected at 800oC.2 Page 3 * Field Effect Transistor MESFETs were considered as the first transistors to try because of unipolar behavior of diamond as semiconductor. The first transistor consisted of a p-channel (channel length 2 µm) B-doped layer deposited on intrinsic diamond crystal. Many devices were fabricated to achieve saturated (“pinch-off”) I-V characteristics. The pinch-off characteristics and high transconductance were obtained for FETs with hydrogenated (with hydrogen atoms chemically bonded) diamond surface. *p-n junction Application of elaborate procedures for P-doping and established procedures for B-doping resulted in fabrication of p-n junction on (001) diamond surface. According to new measurements the P donor level is located at 0.58 eV. UV radiation is emitted by current-injection at room temperature. CHALLENGES IN THE DEVELOPMENT OF DIAMOND ELECTRONICS 1. Shallow Doping The most critical and at the same time the most challenging issue in the diamond program was the control over doping during film growth. Ion implantation and diffusion were also studied b ut the results were not satisfactory. The fundamental requirement for the success of diamond electronics is a possibility of introduction of dopant atoms, at the substitutional positions in the diamond lattice, in such a manner that they create SHALLOW LEVELS, p- or n-type, in the energy band gap. To make a device work efficiently these levels should be shallow enough to assure dopant activation at room temperature, where kT=0.025eV. An example of shallow acceptor in Si is boron (B), its energy level is at 0.045 eV above the edge of the valence band. The shallow donor in Si is phosphorous (P) at 0.044 eV below the edge of conduction band. In diamond B level is at 0.36 eV. As a result of such high ionization energy of boron only 0.2 % holes are activated at room temperature. At higher temperature the device conductivity increases, but the mobility of holes decreases. This behavior limits performance of devices at high frequency. In the case of P situation is even worst, because the P donor is even deeper than B acceptor. The literature data for P donor spread over the range of 0.5 to 1.0 eV. The value of 0.58 eV was reported quite recently. An unfortunate observation regarding diamonds electronics status quo is that the shallow dopants for diamond have not been found. This is a disappointing conclusion for the entire effort spreading over the decades of research. It means that it is not possible at this moment to fabricate diamond electronic devices similar to those made using doped Si or Ge. The reasons, for which finding of the adequate shallow dopants for diamond is a very complex matter, is two fold. First, chemical bonding of carbon atoms in diamond structure is the strongest among all crystal phases. Incorporation of a foreign atom in diamond lattice creates a chemical bonding which is weaker than tetrahedral bonding between carbon atoms. The choice between the candidates for substitutional doping is limited for chemical reasons. Thus, the lattice energy fit to create a shallow dopant level is not possible within this group of elements that could possibly act as dopants. This conclusion was reached after an extensive research, which combined crystal chemistry approaches and first principle quantum mechanics calculations of binding energy of a foreign atom in diamond lattice. Phosphorus, lithium and sodium were among the candidates. None of these studies were successful. The second reason is a disturbance of growth process caused by the presence of foreign atoms on the surface of the growing diamond film. The constructive role of the foreign atom is fulfilled when this atom is incorporated in the substitutional position in the diamond lattice as donor or acceptor. The foreign atom plays a destructive role when its involvement in crystal growth processes results in a creation of growth defects. Unfortunately, in the case of diamond growth minimization of the latter effect is very difficult. The specific situation appears when foreign atoms (or complexes) are active on growing diamond surface, but they are not incorporated into diamond lattice. The important is behavior of nitrogen intentionaladdition to methane/hydrogen mixture. Such N addition at small concentrations ( few atomic %) acts as“gas catalyst”, it means, the growth rate of diamond increases about one order of magnitude. The opposite situation, in active participation, takes place at high concentration of N. The growth rate decreases comparing to pure methane/hydrogen mixture. The material obtained from such process has a distorted diamond structure, which depart from cubic diamond, according to Raman spectra and X-ray diffraction. It is better to name this phase: disordered tetrahedral carbon. Similar distortions of diamond lattice appeared during experiments on doping using P, S and O. Experimental and theoretical study of n-type doping of diamond was the corner stone in the exploration of diamond electronics. Substitutional nitrogen (N) extra deep level lays 1.5 eV below the bottom ofconduction band. The other N atoms create a series of complex defects. Substitutional P atoms create a deep level at around 1 eV. Also, the growth defects create traps inside the band gap, they accompany Pdoping process. The intensive study of doping with sulfur (S) was not successful, as sulfur was found to introduce a lot of growth defects. Other elements were also unsuccessfully tried. The shallow n-doping of diamond is a failure as we see it today. As such it is among key reasons for the lack of adequate progress in diamond electronics. 2. Growth Defects Formation of growth defects during diamond Plasma Enhanced CVD ( PECVD) processes is inherently connected to the interactions with hydrogen, silicon, and metal impurities from the plasma reacting with the wall of deposition chamber. The growth defects have a variety of forms, e.g. hexagonal stacking, twins:(111) and linear (001), interstitial atoms, hydrogen atoms, nitrogen atoms and complexes. They disturb periodicity of diamond lattice. Preserving lattice periodicity is the basic requirement for a scattering-free transport of carriers in semiconductor crystal. This lattice property is explained by Bloch theorem. The crystal lattice of PECVD diamond crystals depart far from strict lattice periodicity. There are areas where local symmetry differs from this of cubic diamond. This is a drastic disturbance of diamond lattice periodicity that affects carrier transport and decreases carrier mobility.3. Homoepitaxy Deposition of electronic quality diamond thin films for fabrication of devices works relatively well on single crystal diamond substrates. By definition, however, homoepitaxy requires as a substrate a defect-free natural or synthetic HP/HT diamond. Any structural defect in the substrate would have an adverse effecton the growth process. For example, inclusion at the surface can initiate growth of a hillock of polycrystalline disordered diamond. Such hillock (e.g. 10 µm across), during doping with B, will create a conductive channel locally introducing short-circuit in the transistor. In practice, homoepitaxial deposition plays a limited role in the research of diamond active electronics because of the difficulty in there producible growth, small area (4x4 mm) substrates, and their prohibitively high price. The additional costis accrued when the growth needs to be carried out on disoriented (001) substrates. The selection of required off-angle substrates relies on a tedious and costly X-ray diffraction characterization performed on the large set of plates. The best results for step flow growth process were obtained when tilt from (001) plane is parallel to <110> direction. The alternative option for step flow growth mechanism is operation at the border of growth/etching, it means at very low methane concentration in hydrogen, (e.g. less than0.1%). At such concentration the population of growth radicals on terraces on (001) surface is low and the choice of specific tilt is not critical. The novel approach is exploration of high temperature growth in respect to roughening transition temperature, which differs for basic crystallographic directions. The second task for homoepitaxy is preparation of CVD substrates for future epitaxial depositions. Such substrates should be less costly than present natural and HP/HT products. A big step forward has been made recently with the announcement by Element Six Ltd of the availability of CVD diamond plates (4.5mm) for cutting tool applications. The hope for scaling up this process is reasonable. 4. Heteroepitaxy Providing adequate quality diamond films are obtained, the heteroepitaxial growth could be a solution for active diamond electronics. The true diamond heteroepiaxy was demonstrated on B terminated (111) surfaces of cubic BN. These HP/HT bulk crystals are not easily available, and their size of 1-2 mm, make fabrication of commercial devices impossible. The exact (in the sense of semiconductor technology)macroscopic heteroepitaxy on other substrates was not achieved as yet. As an alternative, growth ofdiamond polycrystalline, ori ented films was intensively studied. The most preferred choice was orientedgrowth on (001) Si. The growth of preferentially oriented diamond crystals (5-10 µm) was difficult tocontrol. Approximately 90% of crystals have <001> orientation, the rest was arbitrarily oriented. Very local(100 nm) heteroepitaxial growth on (001) Si is documented using High Resolution TEM images. Thelargest uniform area of oriented growth was 25 mm in diameter, but it was smaller when grown in otherlabs. The diamond crystals in such films depart from cubic diamond structure. Silicon (etched bymethane/hydrogen plasma) is incorporated into diamond lattice at interstitial positions as Si-vacancycomplex. Si is easy to detect using electron microprobe and the complex is marked as strong luminescencepeak. Such complex probably causes creation of local linear (001) twins. A new configuration of carbonatoms at the twin line introduces an intermediary sp2/sp3 bonding, which relates to non-diamond band inRaman spectra. The overall X-ra y diffraction and Raman spectra data show lattice disorder, whichdecreases hole mobility. The devices fabricated on this material have inferior parameters comparing tothese fabricated using homoepitaxial film s.The interest in heteroepitaxial deposition of diamond continues, following a limited success with nickelsingle crystal. At present, there is a push toward exploration of other single crystals of metals , e.g. iridiumand related sandwiches, but the progress in this area is slow.ACTIVE ELECTRONICS: CONCLUDING REMARKSRealistic assessment of the progress in the field of active diamond electronics and photonics is that theintensity of R&D in this area did decrease significantly after 1995. Neither electronics nor photonicpractical active devices have been successfully developed. Diamond is an indirect-gap semiconductor, andthe efficiency of luminescence from light emitting diode made with diamond is very week comparing to thedirect-gap GaN devices.Research of single crystal growth was only in part motivated by the needs for substrates for homoepitaxialgrowth of semiconductor diamond films; other demands are coming from jewelry, high pressure research,where Bridgeman anvils need to be improved, single crystal tools like surgical knives or for cutting andetc., etc., etc…. The effort was not entirely in vain, as exemplified by the impressive progress in bulkdiamond crystal CVD growth. At this moment single crystal growth is at the promising stage of development. 1 cm3cubes of diamond have been demonstrated at Carnegie Institution, and 4.5 mm platesare available from Element Six Ltd. Commercial availability of such wafers should be considered as an important breakthrough in diamond wafer engineering.The increase of knowledge about defects in diamond resulted in enhanced capabilities regarding colormanipulation. For instance, imperial red colored diamond crystals were obtained. While this is mainly agemology domain, resulting improved understanding of defect engineering in diamond will certainly advance understanding of optoelectronic properties of diamond. -------------------------------------------------------------------------------- Page 6 The overall good news is that despite of the discussed above difficulties the research on diamond active electronics is continued in many laboratories around the world. The issues of doping, heteroepitaxy andnovel concepts for electronic devices, detectors, and sensors are still in pursuit. For example, the concept ofmonolayer B delta-doping (a pulse doping) for FET design has been reconsidered recently. Metallic type ofconductivity can be obtained using B-doping. The wide range of diamond electrical conductivity, possiblyachievable, makes applications like electrodes, resistors, and heaters seem realistic. It should be noted thatthe electronic band structure of diamond changes with a doping level. Optical spectroscopy has been used to study changes in the density of states in the band gap as a function of doping level. In particular, ofinterest was a determination which model better describes changes in the band structure when theconcentration of B dopant increases: the Bardeen model, which involves changes in the position of thevalence band, or the Mott model, which involves creation of electronic levels around the Fermi level and above the valence band. The results of the analysis indicate that the Mott model, which also can describes the transition to metallic conductivity, better represents physical phenomena involved. Electron field emission from diamond films and tips was extensively studied during last decade. Negative electron affinity and ballistic-like electron transport in the conduction band have been considered as possible explanations for measured currents, which appear in cold cathode devices. PASSIVE ELECTRONICS AND MECHANICAL DEVICESWhile usefulness of diamond in active electronic and photonic device technology is still questionable, its key role in passive electronics and select mechanical device systems is unquestionable. This is because thermal conductivity of diamond is the highest among all solids and it also features exceptional mechanical properties.Diamond crystal conducts heat through elastic waves running across its lattice. This is pure lattice conductivity. The numerical range of measured thermal conductivity is wide, depending on crystal lattice perfection. The highest values have been obtained for mono-isotope pure crystals. High crystallographicquality diamond crystals were grown by HP/HT method for C12, C13and intermediary composition solid solutions of C12and C13, using CVD starting materials. The thermal conductivity of these crystals exceeds thermal conductivity of pure copper 10 times. The thermal conductivity of CVD polycrystalline plates is 3 times higher and single crystal epitaxial films is 5 times higher than in copper. Taking advantage of the exceptional thermal conductivity of diamond, a new concept of IC packaging is under study. Silicon on diamond (SOD) allows placing diamond heat spreader just under the heat emanating transistor. This approach requires deposition of Si active layer on diamond. The benefit is that in this configuration transistors in the circuit are in the immediate proximity of the diamond heat sink. The enhancement of thermal management in next generation ICs is the goal behind this project. As far as mechanical properties are concerned it can be safely stated that diamond crystal is the world best spring. Exploiting elastic properties of diamond, membranes were fabricated by CVD process and applied to high performance speakers. These new generation speakers are available on the market now.Furthermore, the acoustic waves in diamond lattice are transmitted at the lowest attenuation in diamond crystal. This property was exploited for fabrication of high frequency surface acoustic filters. As the base in these filters polycrystalline CVD diamond plates were used. Please direct comments and questions regarding this contribution to: Andrzej Badzian 6

Conclusions and Recommendations There are great opportunities for wide bandgap semiconductors to improve the performance of many nonelectronic technologies. Major benefits to system architecture would result if cooling systems for components could be eliminated without compromising system performance (e.g., power, efficiency, speed). The existence of commercially available high-temperature semiconductor devices would provide significant benefits in such areas as: · sensors and controls for automobiles and aircraft; · high-power switching devices for the electric power industry, electric vehicles, etc.; and · control electronics for the nuclear power industry. With the possible exception of LEDs, however, present commercial demand for wide bandgap semiconductor materials is limited. While there are few pressing applications that cannot be achieved without wide bandgap materials, the vast array of applications, and hence the value, will only be realized once these materials have evolved to such an extent that off-the-shelf devices are available. This chapter is divided into two sections. The first section presents general conclusions and recommendations about future research priorities to accelerate the acceptance of high-temperature semiconductor materials. This section discusses the temperature ranges for the different materials to be used, the competitiveness of U.S. research versus foreign competition, the systems in which high-temperature electronic materials should initially be introduced, and the government/industry/un~versity collaborations required to forward the development of high-temperature semiconductor materials. The second section discusses the barriers to the successful 65 development, manufacture, packaging, and integration of wide bandgap materials into existing systems and presents the key research and development priorities to overcome these barriers. GENERAL CONCLUSIONS AND RECOMMENDATIONS Temperature Ranges Silicon and silicon-on-insulator (SOI) electronics may be sufficient for some applications for temperatures up to 300 °C. Such applications include digital logic, some memory technologies, and some aerated analog and power applications. Silicon-based technology will not be sufficient for many applications operating in the 200- 300 °C range, however, such as power-conditioning devices in higher-temperature control systems. These devices will have to be produced from another material system. Devices based on SiC are well positioned to meet this need, particularly e-channel enhancemer~t-mode MOSFETs. However, significant technological barriers, such asmicropipes, oxide qualify, contacts, metallization, packaging, and reliability evaluation still need to be further addressed. As a result of fundamental limitations, silicon-based technologies will not be useful at temperatures above 300 °C. Other materials must be used for these temperature ranges, but the choices are somewhat less clear. Technology based on GaAs might be used for systems operating up to 400 °C. Just working at elevated temperatures is not the only concern, however. It is also essential that the devices reliably function over a wide range from very cold (i.e., -20 °C) to very hot (i.e., 400 °C). Based on the evidence presented in this report, OCR for page 66 Materials for High-Temperature Semiconductor Devices devices based on e-type SiC are the only type that currently appear to meet the temperature-range and reliability requirements, but additional development is needed. Eventually, high-temperature electronic technology could be developed for reliable operation even for temperatures above 600 °C. U.S. Competitiveness As described in the Preface, considerable international resources are currently being devoted to developing electronic technologies either tailored for or supportive of high-temperature operation. The United States is focusing most of its efforts on high-temperature applications and currently has a slight lead in SiC research. Europe appears to be increasing its effort in wide bandgap materials, especially for power electronics. This research area is synergistic with high-temperature applications because the generation of internal heat is a limiting factor in power devices and can be mitigated by larger bandgap and higher thermal conductivity materials. The dedication of European resources to this area is seen in the founding of the collaborative organization HITEN, which was established in 1992 to coordinate nascent European efforts in high-temperature electronics. Japan is emphasizing the use of wide bandgap materials for opto-electronics and leads in the use of nitrides for light sources. Japan is also becoming interested in power and high-temperature applications. Unfortunately, the closed nature of Japanese industry made it difficult for the committee to determine the true level of interest in wide bandgap materials research. The increased interest in high-power, high-temperature applications is evident in Japan's annual domestic SiC conference, however. The Third Domestic (Japan) SiC Conference convened in Osaka on October 27-28, 1994, with approximately 160 experts in attendance. Contrary to Japan's previous two conferences, there was a greater emphasis at the Osaka conference on high-power, high- temperature applications than on LEDs. The Commonwealth of Independent States had a number of major programs in SiC development, but the current iFinanciall difficulties of most of the Commonwealth's institutions are preventing many laboratories from continuing their research. There is a wealth of expertise and information available for leveraging by other countries, however. For instance, the 66 European Community is planning on supporting a SiC growth effort in St. Petersburg (Y.M. Tairov and V.E. Chelnekov, personal communication, 1994~. The committee believes that the U.S. wide bandgap materials research community is currently very competitive in the international research community. To remain competitive in the international research community, the committee recommends that demonstration technologies be pursued to motivate further research and increase interest in high-temperature semiconductor applications. Demonstration Technologies To increase interest and motivate further research in wide bandgap materials, a realistic, inspiring application focus must be found that can make system designers aware of the benefits of high-temperature electronics. A wide bandgap transistor that operates at 150 °C will not drive the technology because it will be in direct competition with the more economically efficient silicon technologies. The demonstration technologies must be system circuits (i.e., not an individual device) that can be inserted into essentially nonelectronic systems (e.g., turbine engine, nuclear reactor, chemical refinery, or metallurgical mill) with the goal of measurably increasing system performance. As discussed in Chapter 1, the committee believes that there eventually will be a niche market for semiconductors with temperature capabilities higher than that of silicon, and that this market will be sufficiently large to justify the cost of development. However, this belief is tempered by the recognition that because such electronics will be used in new ways there is little immediate demand. The market will grow only in synergy with the availability of components. This suggests that development of high-temperature electronics not be undertaken in isolation. Instead, such development can and should be leveraged from development of other technologies with more immediate applications, thus reducing the costs and the risks of both. Three suitable application areas are high-power electronics, nuclear reactor electronics, and opto-electronics. Power switching devices, for example, would be a good demonstration technology for high-temperature semiconductor materials. High-voltage, high-power electronics, while not necessarily used as high-temperature OCR for page 67 Conclusions and Recommendations devices, nevertheless need wide bandgap semiconductors because of their superior breakdown voltages and high thermal conductivities. There is already considerable research being pursued in this area because (1) improved high-power switching devices could save an estimated $6 billion in the cost of construction of additional transmission lines; and (2) the smoother, more efficient use of the transmission system would reduce the need for new generating capacity, which the Electric Power Research Institute estimates would be a savings of $50 billion in North America alone over the next 25 years (Spitznagel, 1994~. The pursuit of demonstration technologies would not only increase interest in wide bandgap materials, it would also provide significant testbeds for the application of the technology and enhance our understanding of the generic technologies recluired to further high-temperature device operation (e.g., materials etching and implantation; degradation modes of metallic gates, contacts, and interconnects at high temperatures; packaging behavior at high temperatures; and accelerated-testing and reliability- testing methodologies to ensure proper functioning). The ability to grow a reasonably defect-free material is not the only requirement for the realization of a successful technology. The development of demonstration technologies would also help identify other factors that must be resolved for high-temperature electronics to be incorporated into existing systems. Funding Strategy The need for new development funds for demonstration technologies and future wide bandgap materials is not necessary in the comunittee's opinion. Government funding currently exists for long-range research in wide bandgap materials, although additional funding would certainly allow more options to be evaluated within a shorter period of time. Industry has also demonstrated a willingness to commercialize new developments if the projected payback to their investments can occur within the short term (NRC, 1993~. The committee believes that the high-temperature research community should leverage the research funding for wide bandgap materials that Is currently being provided by the high-power and optics markets, where no viable alternatives to wide bandgap materials currently exist. 67 Building on the funding for other areas dependent on wide bandgap materials reduces the need for potential users of high-temperature devices to fund the required materials development exclusively and, thus, may render it cost effective. The committee recommends the following strategy for the development of wide bandgap materials: develop precompetitive alliances and integrated programs (national laboratories, universities, and industries) for coordinating research, technical skills, and capabilities to expedite research in the most efficient manner; direct research at a technology demonstrator that has definite applications (i.e., is a product) and addresses the usually neglected areas of packaging, assembly, testing, and reliability (e.g., high-power switches; integrated motor control; power phase shifter); concurrently develop materials, design, testing, and packaging; and build and test the demonstration component on a cost-share basis that encourages teaming, ensures adequate funds, and requires periodic deliveries. The committee believes that the founding of a newsletter that provides a summary of published worldwide developments in high-temperature semiconductor research would assist the establishment, development, am maintenance of (1) a fundamental long-term materials effort, (2) an infrastructure within the industry, (3) a group to monitor interrzational development, arid (4) a U.S. information group for highlighting advances. MATERIALS-SPECIFIC CONCLUSIONS AND RECOMMENDATIONS The first three parts of this section concentrate on the major wide bandgap materials discussed in this report: SiC, nitrides, and diamond. The final part of this section concerns the generic problems in packaging that will affect the production of all high-temperature electronic O c .evlces. OCR for page 68 Silicon Carbide SiC is an indirect bandgap semiconductor and has enjoyed the longest history and greatest development with regard to both materials growth and device realization. As such, SiC is currently the most advanced of the wide bandgap semiconductor materials and in the best position for near-term commercial application. Its main application will be in high-power, high-temperature, high-frequency, and high-radiation environments. It will not be suitable for blue lasers or ultraviolet light emitters, however, except as a potential substrate material. The specific technical issues for SiC that require further research are summarized in the box, Technical Issues for SiC. The three key research efforts for the development of commercially viable SiC devices are Wafer production: The 1- and 2-inch SiC wafers now in production are rapidly approaching device quality where they might be used for commercial production of devices and circuits with acceptable yield. It could be argued that such small wafers are entirely sufficient for what will be a relatively small market (compared with silicon) with a very high-price premium, and therefore an early investment in larger wafers is not justified. However, the entire commercial infrastructure for electronics manufacture is based on a wafer size of at least 3 inches, and Technical Issues for Nitrides Substrate development Nitride substrates for CVD homo-epitaxy High thermal conductivity, quasi-lattice matching substrates (bow high electrical conductivity and semi-insulating) Further improvement of crystal perfection and doping (CVD grown) Reduce defect density and background impurities Better control of n- and pipe doping New technologies for epitaxial growth Improve surface morphology Improve processing (CVD growth) Ohmic contacts Low contact resistance High-temperature contacts High-temperature packaging Improve understanding of basic properties and knowledge of design parameters Materials for High-Temperature Semiconductor Devices Technical Issues for SiC Purger improvement of crystal perfection (boule and CVD growth) Eliminate micropipes Reduce defect density Reduce background impurities Improve surface morphology Further improvement of doping (boule and CVD growth) New n- and p-type dopants New mid-gap impurities for semi-insulating substrates Introduce rare earth elements in growth Improve processing (boule and CVD growth) Improve oxides/passivation Find alternative insulators (nitrides) Reduce contact resistivin,r for pipe material Develop high-temperature n- and p-type contacts High-temperature packaging Improve understanding of basic properties and knowledge of design parameters preferably 4 inches, as a minimum. Reconstructing a small-wafer infrastructure that became obsolete over 30 years ago will be both an expense and an obstacle to the introduction of commercial SiC electronics. The committee believes that the development of larger SiC wafers is viewed as the more cost-effective approach to commercial development. Film growth: Chemical vapor deposition, molecular-beam epitaxy, and other film-growth technologies and chemistries require refinement to produce epitaxial films with n- and p-type doping ranges from 10~3 to 102° cm~3 for nitrogen, aluminum, boron, gallium, transition metals, and rare earth elements. Manufacturing processes: Lower-cost device- production methods are required to make the manufacture of SiC devices more competitive with the silicon technologies. . Nitrides Interest in the direct bandgap nitride materials (i.e., GaN, A1N, AlGaN, and InGaN) has dramatically increased recently because of their optical properties. The materials show great promise and are likely to dominate the visible and ultraviolet opto-electronics market. Nichia's recent bright blue LEDs have already stimulated increased industrial effort (e.g., Hewlett Packard, Spectra 68 OCR for page 69 Conclusions and Recommendations Technical Issues for Diamond Improvement of grown, crystal perfection, and growth Reduce and control impurities of bulk synthetic diamonds Produce large-area (hetero-epitaxy) single-crystal films of diamond on nondiamond substrates at reasonable cost Synthetically produce larger bulk diamond at reasonable cost Improve n- and p-type doping Improve implantation for doping Improve processing Ohmic contacts Low contact resistance High-temperature contacts Hydrogen passivation Improved understanding Dopant diffusion Knowledge of design parameters Diode Laboratories, Xerox PARC) in materials growth, contact metallurgy and reliability, and device reliability and testing, although the materials have defect densities of greater than 10~°/cm2 and the mechanism of photo emission is currently unknown. Heterojunctions in the nitrides also hold promise for higher-speed devices compared with SiC. Their applicability for power development and nlgn-~requency devices is unproven at this time, and the technologies for wafer production, doping, and etching are currently less developed than SiC and require more longer-term research before they will be competitive with other electronic materials. However, as development of photonic applications for wide bandgap materials progresses, the opto-electronic market may provide an effective way to leverage the development of these materials for high-temperature device applications. The specific technical issues for nitrides research is summarized in the box, Technical Issues for Nitrides. The committee identified the following three research efforts as being key to the development of nitride devices: · Compatible substrates: Better-matched substrates are required for nitride wafer production to be commercially tenable. · Wafer production: Growth of quasi-crystalline films of GaN, AlGaN, and A1N should be pursued on substrates such as SiC to gain thermal advantages. · Doping: Methods for both n- and p-type doping of Group III nitrides are required. 69 Diamond Diamond is a well-understood material, but its use for active electronic device applications is not feasible at this time because of the difficulties associated with its economical growth and doping. While diamond transistors have been designed, fabricated, and tested, their performance is also orders of magnitude less than that which is expected from the electrical properties intrinsic to diamond. The poor performance is thought to result from excessive nitrogen impurities and from as yet not fully explained surface-depletion effects. The current prognosis for diamond is primarily as a protective coating, a thermal management film, and a material for electron-emitting cathodes. The specific technical issues for diamond research are summarized in the box, Technical Issues for Diamond. Packaging Much more research is required in the area of high- temperature packaging. For high-temperature electronics to be commercially viable and provide true performance advantages, interconnection and packaging technologies are required that can reliably operate at temperatures up to 600 °Cfor 1(74 hours. To attain these goals, innovative packaging techniques will be required. The specific technical issues for packaging research are summarized in the box, Technical Issues for Packaging. The three key research efforts for the development of high-temperature packages are · Metallization: Contacts are required in the 10-6 to 10-7 Q/cm2 range that have lon~-term durability at temperatures up to 600 °C. Greater understanding Is needed of the long-term effects Technical Issues for Packaging Improve reliability of high-temperature contacts Improve metallization Improve device development tools Improve process-control tools Improve polishing, cutting, mounting, and etching mends Develop reliability and aging tests Develop computer-aided design tools.
Synthetic diamond, also called lab-created, manufactured, "lab-grown" or cultured diamond is a term used to describe diamond (the tetrahedral carbon allotrope) which has been produced by a technological process, as opposed to natural diamond, which is produced by geological processes. Synthetic diamond is not the same as Diamond-like Carbon, DLC, which is amorphous hard carbon, or diamond simulants, which are made of other materials such as cubic zirconia or silicon carbide. The properties of synthetic diamond depend on the manufacturing process used to produce it, and can be inferior, similar or superior to those of natural diamond.[1] Because it can be made for less than it costs to mine natural diamond, synthetic diamond is used in many industrial applications. Reduced costs and the ability to engineer its physical and electrical properties give synthetic diamond the potential to become a disruptive technology in many areas such as electronics and medicine.
The idea of making less expensive, gem-quality diamonds synthetically is not a new one. H. G. Wells described the concept in his short story "The Diamond Maker," published in 1911. In Capital Karl Marx commented, "If we could succeed, at a small expenditure of labour, in converting carbon into diamonds, their value might fall below that of bricks". a letter refuting an early attempt to create a synthetic diamond.Ever since the discovery that diamond was pure carbon in 1797 many attempts were made to alter the cheaper forms of carbon - generally with little success. One of the early successes reported in the field was by Ferdinand Frédéric Henri Moissan in 1893. His method involved heating charcoal at up to 4000 °C with iron in a carbon crucible in an electric furnace, in which an electric arc was struck between carbon rods inside blocks of lime. The molten iron was then rapidly cooled by immersion in water. The contraction generated by the cooling supposedly produced the high pressure required to transform graphite into diamond. Moissan published his work in a series of articles in the 1890's. Many other scientists tried to replicate his experiments. Sir William Crookes claimed success in 1909. Ruff claimed in 1917 to have reproduced diamonds up to 7 mm in diameter, but later retracted his claims. [4] In 1926, Dr. Willard Hershey of McPherson College read journal articles about Moissan's and Ruff's experiments and replicated their work, producing a synthetic diamond. That diamond is on display today in Kansas at the McPherson Museum. [5] Despite the claims of Moissan, Ruff, and Hershey, many other experimenters had enormous difficulty in creating the required temperatures and pressure with similar equipment, leading some to contend that the early successes were the result of seeding by good-willed co-workers. The most definitive duplication attempts were performed by Sir Charles Algernon Parsons. He devoted 30 years and a considerable part of his fortune to reproduce many of the experiments of Moissan as well as those of Hannay but also adapted processes of his own. He wrote a number of articles -- one of the earliest on HPHT diamonds -- in which he claimed to have produced small diamonds. However in 1928 he authorized C.H Desch to publish an article in which he stated his belief that no synthetic diamonds (including those of Moisan and others) had been produced up to that date. In fact he found that most diamonds produced so far were more likely than not synthetic Spinel. The GE Diamond Project The first person who grew a synthetic diamond according to a reproducable, verifiable and witnessed process was Howard Tracy Hall while working for General Electric in 1954. He received a gold medal of the American Chemical Society in 1972 for his work. [4] In 1941 an agreement was made between General Electric, Norton and Carborundum to further develop diamond synthesis. However this project soon thereafter ended because of the Second World War. They were able to heat Carbon to about 3000 °C (5432 °F) under a pressure of half a million psi, for a few seconds. In 1951 the project was resumed at the Schenectady Laboratories of GE and a high pressure diamond group was formed with F.P. Bundy, H.M. Strong, and shortly afterwards joined by H. T. Hall and others. Following on the work done by Percy Bridgman (who received a Nobel prize for his work in 1946) Bridgman's Anvils were further improved first by Bundy and Strong and later by Hall. The GE team used a tungsten carbide "anvil" within a hydraulic press to squeeze the carbonaceous sample held in a catlinite container, the finished grit being squeezed out of the container through a gasket. It was believed that on occasion a diamond was produced, but since experiments could not be reproduced, such claims could not be maintained. [4] Finally Tracy Hall managed the first commercially successful synthesis of diamond on December 16, 1954 (announced on February 15, 1955). Hall's breakthrough was using an elegant "belt" press apparatus which raised the achievable pressure from 6 to 18 GPa and the temperature to 5000 °C, using a pyrophyllite container, and having the graphite dissolved within molten nickel, cobalt or iron, a "solvent-catalyst". Hall was able to have co-workers replicate his work and the discovery was published in Nature. The largest diamond produced by Hall was 150 micrometres across, clearly unsuitable for ornamentation but very useful in industrial abrasives. [4] This gave rise to an industrial diamond industry that was for decades represented by two main players: GE Superabrasives and De Beers Industrial Diamonds. Another successful diamond synthesis was produced on February 16, 1953 in Stockholm, Sweden by the QUINTUS project of ASEA (Allemanna Svenska Elektriska Aktiebolaget), Sweden's major electrical manufacturing company using a bulky split sphere apparatus designed by Baltzar von Platen and the young engineer Anders Kämpe (1928–1984). Pressure was maintained within the device at an estimated 83,000 atmospheres (8.4 GPa) for an hour. A few small crystals were produced, but not of gem quality or size. The work was not reported until the 1980s. During the 1980s a new competitor emerged in Korea named Iljin Diamond, followed later by hundreds of Chinese entrants. Iljin Diamond allegedly accomplished this by misappropriating trade secrets from GE via a Korean former GE employee in 1988 (General Electric v. Sung, 843 F. Supp. 776). In 2003 GE sold GE Superabrasives to a private equity firm called Littlejohn and it was renamed to Diamond Innovations. Littlejohn sold Diamond Innovations to Sandvik in January of 2007. Also, in 2002, De Beers Industrial Diamonds rebranded to Element Six and is operating as an independent company from De Beers. Many more companies have become important players in the industrial diamond market. The main ones are Sumitomo Electric Hardmetal, US Synthetic, Smith Megadiamond and Novatek. Some smaller companies have signaled their intent to enter the market for gems using synthetic diamond. These are Adia Diamonds, Apollo Diamond, Gemesis and Tairus and a German company("SEDKRIST). As of 2006, the industrial diamond industry is an annual US$1 billion market, producing some 3 billion carats, or 600 metric tons, of synthetic diamond. This should be put in comparison with the 130 million carats (26 metric tons) mined annually for gem purposes. Synthetic gem-quality diamond crystals were first produced in 1970 (reported in 1971) again by GE. Hall had continued to work for GE, developing the tetrahedral press with four anvils. Large crystals need to grow very slowly under extremely tightly controlled conditions. The first successes used a pyrophyllite tube seeded at each end with thin pieces of diamond and with the graphite feed material placed in the centre, the metal solvent, nickel, was placed between the graphite and the seeds. The container was heated and the pressure raised to around 55,000 atmospheres. The crystal grow as they flow from the centre to the ends of the tube, the longer the process is extended the larger the crystals - initially a week-long growth process produced gem-quality stones of around 5 mm and one carat. The graphite feed was soon replaced by diamond grit, as there was almost no change in material volume so the process was easier to control. The first gem-quality stones were predominantly cubic and octahedral in form and, due to contamination with nitrogen, always yellow to brown in color. Inclusions were common, especially "plate-like" ones from the nickel. Removing all nitrogen from the process by adding aluminium or titantium produced a colourless 'white' stone, while removing the nitrogen and adding boron produced a blue. However removing nitrogen slows the growth process and impairs the crystals properties, so most stones are still yellow. In terms of physical properties the GE stones were not quite identical to natural stones. The colourless stones were semi-conductors and fluoresed and phosphoresed strongly under SWUV but were inert under LWUV - in nature only blue stones should do this. All the GE stones also showed a strong yellow fluorescence under X-rays. De Beers Diamond Research Laboratory has since grown stones of up to 11 carats, but most stones are around 1 to 1.5 carats for economic reasons, especially with the spread of the Russian BARS apparatus since the 1980s. The GE method is called HPHT (High Pressure, High Temperature), but there is another method. Following on from work by John Angus and Boris Spitsyn researchers at the National Institute for Research in Inorganic Materials in Tsukuba produced diamonds at less than one atmosphere of pressure and only 800 °C through CVD, Chemical Vapour Deposition. The Japanese had begun their research in 1974 and reported their success in 1981. The Japanese passed a mixture of carbon-containing gas (methane in their case) and hydrogen into a quartz tube at a pressure of 0.05 atmospheres. Using microwaves the mixture was heated to 800 °C, disassociating both the methane and hydrogen into elemental forms. The carbon is deposited on a substrate, the majority as graphite but a very small proportion as diamond crystal. the graphite is 'removed' by the hydrogen leaving a thin layer of diamond, initially the layer was around 25μm in thickness. Properties of synthetic diamond The gem diamond most people are aware of is just one of many different forms that diamond can take. Natural gem diamond is a single crystal diamond with low levels of impurities. This homogeneity is what allows it to be clear, while its material properties and hardness are what make it a popular gemstone. Most natural diamond removed from the earth's crust does not have the high purity or high crystallinity necessary to be a quality gemstone. Following are some important properties by which various types of diamond are described. Crystallinity A mass of diamond may be one single, continuous crystal or it may be made of up many smaller crystals ("polycrystalline"). Single crystal diamond is typically used in gemstones, while polycrystalline diamond is commonly used in industrial applications such as mining and cutting tools. Within polycrystalline diamond the diamond is often described by the average size of the crystals that make it up, called the "grain size." Grain sizes range from hundreds of micrometers to nanometers, usually referred to as "microcrystalline" and "nanocrystalline" diamond, respectively. Hardness A diamond's hardness can vary depending on its impurities and crystallinity. Nanocrystalline diamond produced through CVD diamond growth, for instance, can have a wide range of hardness from 30% to 75% of single crystal diamond, and the hardness can be controlled to be used in specific applications. Some single crystal diamonds grown through chemical vapor deposition has been shown to be harder than any known natural diamond. Impurities and Inclusions No crystal is absolutely pure. Any substance other than carbon found in a diamond is an impurity, and may also be called an inclusion, due to the way these impurities fall in the crystal lattice. While inclusions can be unwanted, they can also be introduced on purpose to control the properties of the diamond. For instance, while pure diamond is an electrical insulator, diamond with small amounts of boron added is an electrical conductor, possibly allowing it to be used in new technological applications. Manufacturing technologies There are two main methods to produce synthetic diamond. The original method is High Pressure High Temperature (HPHT) and is still the most widely used method because of its relative low cost. It uses large presses that can weigh a couple of hundred tons to produce a pressure of 5 GPa at 1,500 degrees Celsius to reproduce the conditions that create natural diamond inside the Earth. The second method, using chemical vapor deposition or CVD, was invented in the 1980s, and is basically a method creating a carbon plasma on top of a substrate onto which the carbon atoms deposit to form diamond. High Pressure, High Temperature (HPHT) There are two main press designs used to supply the pressure and temperature necessary to produce synthetic diamond. These basic designs are the belt press and the cubic press. There are a number of other designs, but none of them are used for industrial scale manufacturing. The original GE invention by H. Tracy Hall, uses the belt press, wherein upper and lower anvils supply the pressure load and heating current to a cylindrical volume. This internal pressure is confined radially by a belt of pre-stressed steel bands. A variation of the belt press uses hydraulic pressure to confine the internal pressure, rather than steel belts. Belt presses are still used today by the major manufacturers at a much larger scale than the original designs. The second type of press design is the cubic press. A cubic press has six anvils which provide pressure simultaneously onto all faces of a cube-shaped volume. The first multi-anvil press design was actually a tetrahedral press, using only four anvils to converge upon a tetrahedron-shaped volume. The cubic press was created shortly thereafter to increase the pressurized volume. A cubic press is typically smaller than a belt press and can achieve the pressure and temperature necessary to create synthetic diamond faster. However, cubic presses cannot be easily scaled up to larger volumes. To illustrate, one could increase the pressurized volume by either increasing the size of the anvils, thereby increasing by a great factor the amount of force needed on the anvils to achieve a similar pressurization, or by decreasing the surface area to volume ratio of the pressurized volume by using more anvils to converge upon a different platonic solid (such as a dodecahedron), but such a press would be unnecessarily complex and not easily manufacturable. Chemical Vapor Deposition of Diamond (CVD) Main article Chemical vapor deposition of diamond Chemical vapor deposition of diamond is a method of growing diamond by creating the environment and circumstances necessary for carbon atoms in a gas to settle on a diamond substrate in diamond crystalline form. This method of diamond growth has been the subject of a great deal of research since the early 1980s, and since 2003[8] Apollo Diamond has been producing gem-quality diamonds. As of 2007, they produce colourless, near-colourless, cognac and salmon-coloured diamonds.[9] Applications Given the extraordinary set of physical properties diamond exhibits, diamond has and could have a wide-ranging impact in many fields. Wear Resistance Diamonds have long been used in machining tools, especially when machining non-ferrous alloys. While natural diamond is certainly still used for this, the amount of synthetic diamond is far greater. The most common usage of diamond in cutting tools is done by distributing micrometer-sized diamond grains in a metal matrix (usually cobalt), hardening it and then sintering it onto the tool. This is typically referred to in industry as “PCD” diamond. PCD tipped tools are often used in mining and in the automotive aluminum cutting industry. For the past fifteen years work has also been done in the hope of using CVD diamond growth to coat tools with diamond,[10] and though the work still shows promise it has not significantly displaced traditional PCD tools. Electronics CVD diamond also has applications in electronics. Conductive diamond has been demonstrated as a useful electrode under many circumstances. For example, University of Wisconsin-Madison chemistry professor Robert Hamers has developed photochemical methods for covalently linking DNA to the surface of polycrystalline diamond films produced through CVD. Also, the diamonds have been shown to detect redox reactions that can't ordinarily be studied and in some cases degrade redox-reactive organic contaminants in water supplies. Because diamond is almost completely chemically inert it can be used as an electrode under conditions that would destroy traditional materials. For such reasons waste water treatment of organic effluents as well as production of strong oxidants have been published. There are already a number of companies producing diamond electrodes. Diamond has shown great promise as a potential radiation detection device. Diamond has a similar density to that of soft tissue, is radiation hard and has a wide bandgap. These qualities suggest it has potential to be an excellent radiation detection material, and it has already been employed in some applications, such as the BABAR detector at Stanford.[14] Diamond also has potential uses as a semiconductor. This is because the diamonds can be "doped" with impurities like boron and phosphorus. Since these elements contain one more or one less valence electron than carbon, they turn the diamonds into p-type or n-type semiconductors. There are also studies being conducted about impregnating boron-doped CVD diamonds with deuterium to produce n-type semiconducting diamonds. Diamond transistors are functional to temperatures many times that of silicon and are resistant to chemical and radioactive damage. While no diamond transistors have yet been successfully integrated into commercial electronics, they show promise for use in exceptionally high power situations and hostile environments. CVD diamond growth has also been used in conjunction with lithographic techniques to incase microcircuits inside diamond. Researchers at Lawrence Livermore National Laboratory and the University of Alabama, Birmingham use this process to create designer diamond anvils as a novel probe for measuring electric and magnetic properties of materials at ultra high pressures using a Diamond Anvil Cell. HPHT "type IIa" diamonds are, as of 2007, approaching the very high purity and crystallographic structure perfection required to replace silicon in applications like X-ray tomographic imaging at synchrotrons;[17] they will be able to sustain the increased intensities of next generation light sources. Synthetic gems Adia Diamonds, Gemesis, New Age Diamonds and Tairus all produce gems made through HPHT technology. They are grown in split sphere high-pressure, high-temperature (HPHT) crystal growth chambers that resemble washing machines. The device bathes a tiny sliver of natural diamond in molten carbon at 1500 °C and 58,000 atm (5.9 GPa). This produces a rough diamond which can be cut down to a polished size close to half its original carat weight. Gemesis diamonds have an orange tint that is rare in natural diamonds. The yellow tint occurs when approximately five out of each 100,000 carbon atoms in the diamond crystal lattice are replaced with nitrogen atoms. Adia Diamonds produces diamonds in various shades of yellow and orange as well as blue and white (colorless). The blue color comes from doping the diamond with boron, rather than nitrogen, during the growth process. White diamonds must be grown in an environment free of nitrogen and boron, which makes them very difficult to produce. Yellow diamonds are more profitable because they can be made more quickly and cost less to manufacture than blue or colorless diamonds. The mined diamond industry is evaluating marketing and distribution countermeasures to these less expensive alternatives. Gem-quality diamonds grown in a lab can be chemically, physically and optically identical to naturally occurring ones although they can be distinguished by spectroscopy in infrared, ultraviolet, or X-ray wavelengths. The DiamondView tester from De Beers uses UV fluorescence to detect trace impurities of nickel or other metals in HPHT diamonds, or hydrogen in some LP CVD diamonds. Furthermore, all three manufacturers have made public statements about selling their diamonds with full disclosure and have implemented measures to laser-inscribe serial numbers on their gemstones.[8][19] LifeGem is a company offering to synthesize diamonds from the carbonized remains of people or pets.

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