Proc. Nadl. Acad. Sci. USA Vol. 89, pp. 821-826, February 1992 Colloquium Paper

This paper was presented at a colloquium entitled "Industrial Ecology," organized by C. Kumar N. Patel, held May 20 and 21, 1991, at the National Academy of Sciences, Washington, DC. Alternative starting materials for industrial processes (replacement chemicals/toxic reagent substitutes/on-demand chemical generation) JAMES W. MITCHELL Analytical Chemistry Research Department, AT&T Bell Laboratories, Murray Hill, NJ 07974

ABSTRACT In the manufacture of chemical feedstocks Table 1. High-technology industries based on chemistry and subsequent processing into derivatives and materials, the U.S. chemical industry sets the current standard of excellence Chemicals Materials for technological competitiveness. This world-class leadership Agrochemicals Ceramics is attributed to the innovation and advancement of chemical Electronic reagents Glass engineering process technology. Whether this status is sus- Paints and solvents Metals and alloys tained over the next decade depends strongly on meeting Petrochemical feedstocks Paper increasingly demanding challenges stimulated by growing con- Pharmaceuticals Plastics and rubbers cerns about the safe production and use of chemicals without Soaps and detergents Synthetic fabrics harmful impacts on the environment. To comply with stringent environmental regulations while remaining economically com- U.S. is afforded one ofits few remaining globally competitive petitive, industry must exploit alternative benn starting ma- edge high-technology industries (1). In addition to its positive terials and develop environmentally neutral industrial pro- impact on the economy, the industry is crucial in the main- cesses. Opportunities are described for development of envi- tenance of the high standard of living that is now common- ronmentally compatible alternatives and substitutes for some of place in developed countries. Chemistry plays a key role in the most abundantly produced, potentially hazardous indus- food production, in the supply of materials for clothing and trial chemicals now labeled as "high-priority toxic chemicals." shelter, in preventing disease, and in providing health care For several other uniquely important commodity chemicals products. Chemical technology also is the cornerstone on where no economically competitive, environmentally satisfac- which a number of other materials industries depend, as tory, nontoxic alternative starting material exists, we advocate indicated in Table 1. the development of new dynamic processes for the on-demand Despite the pivotal role of chemistry in advancing tech- generation of toxic chemicals. In this general concept, which nology, the benefits derived are frequently erroneously per- obviates mass storage and transportation of chemicals, toxic ceived to be counterbalanced by a Gaussian distribution of raw materials are produced in real time, where possible, from destructive manifestations. Predictably, misuse, abuse, and less-hazardous starting materials and then cbemically trans- accidents involving chemicals and hazardous materials can formed immediately into the rial product. As a selected produce catastrophic consequences for humans and for the example for semiconductor technology, recent progress is environment (2, 3). However, in a manner somewhat analo- reviewed for the on-demand production of arsine in turnkey gous to the usage of medicine, appropriate chemicals when electrochemical generators. Innovation ofon-demand chemical used properly in correct amounts more often produce the generators and alternative processes provide rich areas for desired beneficial results. This dichotomy of beneficial and environmentally responsive chemical engineering processing destructive impacts of chemicals is exemplified well by research and development for next-generation technology. pesticides and herbicides, by explosives and fuels. Many commodity chemicals used for these purposes are intrinsi- The chemical industry, often maligned and undervalued by cally dangerous to humans and animals. Still other chemical society, has an exemplary past with a key role in elevating the reagents with unique properties can be designed specifically standard of living of developed countries. Currently, U.S. for their lethal properties and exploited because of persistent industry sustains a healthy economic position as one of the stability. Involvement of such hazardous chemicals in envi- very productive high technologies (1). Its future status, ronmental incidents and accidents contributes to the spread although gauged to be continuously prosperous, will be of chemophobia among the general public. Certainly, media imperiled by difficult technical and demanding economic coverage of events such as those listed in Table 2 is news- challenges generated by increasing pressures to transform worthy. Unfortunately, the extensive and disproportionate existing industrial chemical processes into environmentally coverage sometimes given to such incidents, but not reported neutral manufacturing operations (2). Fortunately, U.S. in- for chemical benefits, perpetuates a negative perception of dustry possesses great capacity for technical innovation and that has reinvestments for the future, which are sustained by its chemistry become the more frequently held image economic health. among the average citizen. Apparently, sensational coverage Economically, the industry is unequivocally successful of several maladies can easily overshadow the entire history and through the manufacture of chemical feedstocks, deriv- of tremendous benefits that far exceed the detriments of the atives, and production ofmaterials, has maintained a positive industry. balance oftrade consistently over several decades. Thus, the Abbreviations: EPA, Environmental Protection Agency; HPTC, high-priority toxic chemical; CFC, chlorofluorocarbon; PCB, poly- The publication costs of this article were defrayed in part by page charge chlorinated biphenyl; CVD, chemical vapor deposition; MES-FET, payment. This article must therefore be hereby marked "advertisement" metal semiconductor-field effect; VPE, vapor phase epitaxy; MBE, in accordance with 18 U.S.C. §1734 solely to indicate this fact. molecular beam epitaxy.

821 Downloaded by guest on September 29, 2021 822 Colloquium Paper: Mitchell Proc. Natl. Acad. Sci. USA 89 (1992) Safeguards against the potential dangers associated with Table 3. HPTCs the production and use of chemicals are necessary. Many Benzene Mercury (cpds) chemicals are highly toxic and accumulate in biological Cadmium (cpds) Methyl ethyl ketone systems, in waterways, in soil, and in the atmosphere. Carbon tetrachloride Methyl isobutyl ketone Consequently, federal and state agencies implement and Chloroform Nickel (cpds) enforce compliance regulations to provide environmental Chromium (cpds) 1,1,1-Trichloroethane protection. Historically, the chemical industry has been well Cyanide (cpds) Trichloroethylene policed and monitored. Legislative mandates continue to be Dichloromethane Xylenes issued to protect industrial workers, the general population, Lead (cpds) Dioxins and the planet from the long-term effects of chemical expo- sure. Under the control of the Occupational Safety and Evaluated by the EPA as presenting significant risk to human Health Administration (OSHA), and with the surveillance of health and the environment. cpds, Compounds. the Environmental Protection Agency (EPA), industry in Resource Conservation and Recovery Act (RCRA) and re- general and the chemical industry specifically have embodied cent state supreme court interpretations are establishing a safety as one of the premium parameters of manufacturing. trend that tends to hold previous commercial landowners This emphasis on and cognizance of the need for protection forever liable for the cleanup and for damages resulting from of people within the work place from chemical exposure and and technological activity. other hazards are now being paralleled by an increasing pollution generated by industrial awareness of the vital need to protect the planet from Cradle-to-grave accountability for the generation, treatment, long-term environmental degradation resulting from man- and disposal of hazardous materials appears to be the rule for made chemicals (2). To achieve this end, more stringent U.S. the 1990s and beyond. legislation has been passed recently and international coali- tions are forming to retard global environmental pollution. Technology for Preventing Pollution A great impetus for the "greening of the planet" (environ- mentally consciousness industrialization) is provided by new Scrutiny of the total environmental picture involves an anal- mandates to control air pollutants. The Clean Air Act amend- ysis ofthe primary components compliance, recycling, waste ments of 1990 target 191 air toxic chemicals for reduced treatment, and site remediation. Requirements for compli- emissions. Several of these chemicals have been designated ance are becoming more stringent at the same time that costs as high-priority toxic chemicals (HPTCs) (Table 3). This label for remediation and treatment are skyrocketing. These trends implies that there is a sufficient data base ofexperimental and compel industries to examine and also exploit the economy toxicological information to indict these chemicals as pre- of recycling-based processing. Clearly, approaches to pollu- senting significant risks to human health and the environ- tion prevention must be incorporated into chemical process- ment. Among these compounds are included 1,2-dichloro- ing and manufacture. In view ofrapidly expanding ecological ethylene, other chlorocarbons listed in Table 3, and chloro- consciousness, and the compelling economics of environ- fluorocarbons (CFCs) that catalyze the destruction of mental compliance, innovation of pollution-preventing tech- atmospheric ozone. These chemicals are sufficiently destruc- nology as opposed to add-on waste treatment and remedia- tive to the environment that their large-scale usage is sched- tion technology is the preferred approach for next-generation uled to be either drastically reduced or phased out over the manufacture of chemicals. next decade (1). The Montreal Protocols specifically stipulate Already the U.S. is a world leader in the development of an international agreement to terminate the commercial pro- environmental technologies and in the practice of environ- duction of ozone-depleting CFCs by 2000 (4). mental protection (7). This state of readiness results from More stringent controls of water pollutant chemicals are on enlightened management and the influences of federal com- the horizon as well. The passage of the Safe Drinking Water pliances, which have induced the chemical industry to take a Act, which takes effect in 1992, doubles the number of leadership role in implementing safety awareness. A similar pollutants subject to federally enforceable drinking water track record might well be expected for the industry's lead- standards. The regulation sets maximum contaminant levels ership in institutionalizing environmentally responsive pro- for four widely used pesticides (alachlor, aldicarb, atrazine, cessing and manufacture. To accomplish this new mission, and pentachlorophenol); 13 other pesticides; 10 volatile or- chemicals and materials manufacturing ofthe future will need ganic chemicals; polychlorinated biphenyls (PCBs); and 8 to deliberately develop and exploit new pollution prevention inorganic chemicals including cadmium, nitrate, and nitrite. technology and strategies. Within this regime of operation, Ultimately, -85 contaminants will be regulated by enforce- the industry may be compelled to innovate environmentally able standards by July 1992 (5). responsive processes and procedures to replace existing ones Other legislation is aimed at cleaning up extensively pol- that have required decades to develop. By contrast, envi- luted land and waterways and regulating hazardous materials ronmentally suitable alternatives will need to be innovated and wastes. The Superfund Amendments and Reauthoriza- within a few years. In the achievement of these endeavors, tion Act (SARA) regulates the tracking of extremely hazard- ous chemicals and in tandem with the Toxic Substances Table 4. Chemicals in 1990 top 50 largest volume products Control Act provides the EPA and states with the authority Rank Substance lb x 10-9 The to control the industrial use of hazardous materials (6). 15 Ethylene dichloride 13.30 Table 2. Chemicals and sites involved in incidents or accidents 16 Benzene 11.86 17 Xylenes 10.90 Chemicals Site(s) 18 Vinyl chloride 10.65 Agent Orange Bhopal, India 19 Ethylbenzene 8.99 Lead pigmented paint Love Canal, NY 20 Styrene 8.02 DDT Persian Gulf 23 Formaldehyde 6.41 DES Superfund sites 25 Toluene 6.10 PCBs Valdez, Prince 27 Ethylene oxide 5.58 Thalidomide William Sound 29 Ethylene glycol 5.03 39 Acrylonitrile 3.30 DDT, dichlorodiphenyltrichloroethane; DES, diethylstilbestrol. Downloaded by guest on September 29, 2021 Colloquium Paper: Mitchell Proc. Natl. Acad. Sci. USA 89 (1992) 823 Table 5. Properties of N-methyl-2-pyrrolidone substitute for chloride. Whenever possible, substitutions of an environ- methylene chloride mentally compatible reagent for one that becomes targeted Methylene N-Methyl- for phased-out manufacture is a viable alternative that is Property chloride 2-pyrrolidone easily exploited when no or minimal capital investment is involved. Process changes that eliminate Solvent power Excellent Excellent the use of the Flammability None Flash point 910C targeted chemical are even more valuable. (auto. ign.) 2700C Simple substitutes cannot replace many commodity chem- Solubility in H20 2% (vol/vol) Completely icals that are unique precursors for the synthesis of second- bp 39.70C 2020C ary derivatives and materials. For example, formaldehyde fp -950C -250C and vinyl chloride both have unique properties and tremen- Toxicity Narcotic* Eye, skin* irritant dous industrial value, while also being hazardous to humans. Biodegradable Moderate Where such chemicals do not bioaccumulate, have well Rapid known harmful threshold levels for humans, and are con- State Liquid Liquid trolled by exceedingly stringent regulations that safeguard *Total human toxicity still not unequivocally defined. their storage, transport, handling, and work-place exposure, labeling as HPTCs is inappropriate. It is imperative, how- one expects that chemical substitutes and on-demand pro- ever, that next-generation methodologies be innovated to cesses will receive focused attention as alternatives to exist- further ensure the protection of society and the environment ing technology that pose environmental problems. from "known-to-be-harmful" chemicals. Chemical Substitutes and Replacements On-Demand Chemical Generation The most viable environmentally responsive strategy for In conventional large-scale production of hazardous chemi- HPTCs includes reduced usage, phased-in replacement, and cals, several phases pose environmental risks. As depicted ultimately, where fully warranted, phased-out large-scale schematically in Fig. 1, large-scale chemical processing production. Thus, the production ofenvironmentally suitable plants are engineered to achieve synthesis, separation, and substitutes for HPTCs is an immediate challenge for the temporary on-site storage ofvarious products. Subsequently, chemicals industry. Among the targeted group of chemicals precursor chemicals are transported by appropriate vessels identified in Table 3, several are commodity products man- through the public domain to customer premise sites, where ufactured in high volume. As shown in Table 4, methylene secondary storage provides a stockpile of chemicals for chloride, benzene, and the xylenes rank among the top 20 customer use. For dangerously toxic chemicals, the two chemicals produced in 1990 (8). To preclude economic losses stages of storage and the possibility of accidents during associated with commodity chemicals being phased out, the transport pose acceptable risks that are now dealt with industry is introducing replacements at an astounding pace. relatively easily on a daily basis. Already the substitute, N-methyl-2-pyrrolidone, is being An alternative to conventional processing of extremely commercialized as a promising replacement for methylene toxic chemicals is on-demand generation, which is diagramed chloride. The properties of this environmentally compatible in Fig. 2. Conceptually, this approach involves the dynamic chemical replacement are compared in Table 5 with proper- reaction of less-hazardous precursors to generate the more- ties that dictate the industrial applications of methylene hazardous product only when it is needed. In an appropriate CHEMICAL PLANT -_ -_-_ __

CUSTOMER I PREMISE

I.I I RAI**

I II I." I I _v

m - m ------m -mm m i FIG. 1. Conventional production of commodity chemicals. Downloaded by guest on September 29, 2021 824 Colloquium Paper: Mitchell Proc. Nadl. Acad. Sci. USA 89 (1992)

PRODUCTION

ENVIRONMENTALLY SAFE

FIG. 2. Schematic of on-demand generation of commodity chemicals. reactor (R1), the precursors are dynamically reacted to gen- Where large amounts of arsine are required for manufacture, erate a product stream of the desired toxic chemical, which planning for this unlikely event includes storage of arsine in is in turn fed in real time into a chemical process reactor. This remote outdoor or rooftop facilities. These facilities can on-demand generation ofthe toxic chemical at the customer's require expenditures of up to $1 million to accommodate site and its subsequent real-time consumption in manufacture containment and storage and to provide transport lines to the eliminates storage and transport of dangerously toxic chem- point of use in the laboratory. Despite these expenses, the icals. Innovation of on-demand generators for extremely U.S. electronics industry has developed optimized proce- hazardous chemicals is the alternative that might well be the dures for handling toxic gases and has installed state-of-the- approach of the future for chemical products such as vinyl art facilities for containment, storage, recovery, and disposal chloride, formaldehyde, methylisocyanate, phosgene, hydra- of toxic reagents. zine, ethylene chlorohydrin, and other chemicals. The de- Due to the acute toxicity of arsine (more toxic than velopment of large-scale, economic, on-demand generators hydrogen cyanide), investigations have been conducted to interfaced to processes for the real-time consumption of examine alternatives to arsine in cylinders. A possible solu- lethally toxic chemicals in manufacturing could be an ex- tion to the hazard associated with the sudden release ofarsine tremely fruitful venture for the chemical process industry of stored in compressed cylinders is containment of the gas at the 21st century. atmospheric pressure. Investigators at the Naval Research Laboratory have demonstrated atmospheric-pressure stor- Alternative Electronic Chemicals age ofAsH3 in zeolites at 23°C (9). Desorption ofthe adsorbed reagent (-25% by weight) below 190°C was demonstrated Alternative precursor reagents for use in semiconductor along with the growth of GaAs and AlGaAs materials by manufacture are needed to further improve margins ofsafety. vapor-phase epitaxy. This approach eliminates the hazard All of the reagents arsine, silane, dichlorosilane, germane, associated with compressed gas cylinders. However, it does phosphine, and aluminum and indium alkyls are essential not circumvent the limitations imposed by safety guidelines precursors for fabricating electronic devices. These chemi- that restrict storage of arsine to 100 g or less. While the cals, some lethally toxic gases and others pyrophoric, have restriction of the storage of the toxic gas to 100 g at atmo- been supplied in compressed gas cylinders and used safely in spheric pressure enhances safety in the work place, device device manufacture for >3 decades. However, there is the manufacture can be affected negatively. Device yields and ever-present potential hazard associated with the use of reliability are susceptible to fluctuations of reagent quality compressed cylinders. This ultimate concern for safety is the associated with frequent changes oflow-capacity systems. In "sudden release hazard" associated with a catastrophic fact, frequent changes of the arsine source may actually failure ofthe compressed gas cylinder or its associated valve. decrease safety by increasing the opportunity for human error. Table 6. Toxicity data for substituted arsine We have devised other alternatives to arsine in compressed gas cylinders. In principle, the preferred solution is the LC50, replacement ofarsine by completely nontoxic reagents. Prog- Name Formula ppm* Rating ress in developing nontoxic, volatile alternatives to arsine has Arsine H3As 42 Highly toxic occurred. As shown in Table 6, lethally toxic arsine is made Dimethylarsine HAs(CH3)2 164 Moderately toxic increasingly less toxic by alkylation. The fully substituted Trimethylarsine As(CH3)3 >14,027 Practically nontoxic reagent As(CH3)3 is practically nontoxic according to con- t-Butylarsine H2As(Bu-t) 90 Highly toxic temporary chemical standards. In addition, as the substitu- *Lethal concentration in ppm where 50% of test animals expired tion increases, the reagent becomes less volatile. As(CH3)3 is during a 4-hr exposure period. a liquid at room temperature. This fully alkylated methyl Downloaded by guest on September 29, 2021 Colloquium Paper: Mitchell Proc. Natl. Acad. Sci. USA 89 (1992) 825

FEEDBACK LOOP

ON - LINIE PROCES S CONTROL MIONITOR

PRODUCT STREAM FIG. 3. Components of on-dem=land electrochemical arsine generator. analog of arsine, trimethylarsine, has been investigated in C into the GaAs layer is minimized by using the partially detail (10, 11). Although this reagent is useful in pyrolytic alkylated reagent, tertiarybutylarsine, [H2A,(Bu-t)] (12). chemical vapor deposition (CVD) deposition ofGaAs at 4000C Since this material is only somewhat less toxic than arsine, it or more, the resulting film contains considerable levels of still requires precautions in use. Because it is a volatile liquid carbon incorporation (>1016 atoms per cm3). Co-deposition of and not a gas at room temperature, (t-Bu)AsH2 can be handled

FIG. 4. On-demand electrochemical arsine generator. Downloaded by guest on September 29, 2021 826 Colloquium Paper: Mitchell Proc. Nati. Acad. Sci. USA 89 (1992) Table 7. Purity of electrochemically generated arsine nitrogen may slowly effuse from the pores ofthe zeolites used Arsine 83.49% in the drying column. Occasionally, up to 33 ppm of nitrogen Hydrogen 16.50% has been measured. Fortunately, this inert impurity is innoc- Nitrogen 33 ppm uous for all CVD applications of arsine. After verification of the purity of the arsine product, the Other impurities not detectable by highest sensitivity mass spec- generator was interfaced to a hydride VPE reactor. InGaAs/ trometry: H2AsOH, H2AsCH3, BiH3, CO, C02, Ga2H6, GeH4, H2Se, InP MES-FET devices have been fabricated and shown to H20, H2Te, 02, PbH4, PH3, SbH3, SiH4, SnH4. equal or exceed the best performance of identical devices fabricated with arsine from commercially available cylinders more safely than cylinders of compressed arsine gas. A (14). Recently, the generator has been interfaced to and commercial source ofa high-purity product has been identified proven compatible for MBE fabrication of materials. GaAs and used by device engineers to fabricate metal semiconduc- and InGaAs materials were fabricated in tandem using on- tor-field effect transistor (MES-FET) devices that meet cur- demand generated and cylinder arsine. GaAs from the gen- rent-voltage and other performance specifications (13). erated arsine were 103 lower in impurities and InGaAs were To eliminate arsine storage, a technique to generate it a factor of 4 lower in impurities than corresponding materials on-demand in desired quantities and deliver it in real time to produced from an arsine cylinder. Both laser-induced pho- the reactor has been devised (12). The technique is based on toluminescence and current-voltage characterizations con- electrochemical on-demand synthesis of the reagent at an firmed the superior purity of the materials fabricated with arsenic metal cathode in a suitable electrolytic cell containing on-demand generated arsine. 1.0 M potassium hydroxide. Fig. 3 is a schematic of the prototype pilot-scale unit. This unit has a total generating Future Directions capacity of 10 lb of arsine delivered over a wide range of concentrations (2-85% in H2), at selectable pressures up to 60 The development of nontoxic, and environmentally compat- psig, and at variable flow rates up to 1.0 liter/min. These ible alternatives for critically important precursor reagents reagent fluxes meet all requirements for CVD, vapor phase for semiconductor device manufacture is greatly important epitaxy (VPE), and molecular beam epitaxy (MBE) and and worthy of comprehensive investigations. In those cases satisfy most conditions for (MOCVD). where nontoxic alternatives for Si, As, P, In, Al, and Ga A photograph ofthe first commercial on-demand generator reagents cannot be produced, the next best alternative of is producing existing toxic precursors by on-demand turnkey of a precursor for semiconductor device fabrication shown generators must be pursued vigorously. In addition, chemical in Fig. 4. Within the electrochemical cell, a unique packed- engineering research to develop clever methods for large bed cathode compartment permits up to 10 lb of pure arsenic scale on-demand production of hazardous chemicals for to be reduced to arsine. The electrode compartment is immediate conversion into benign products will provide designed for uniform current distribution, thus ensuring opportunities for chemical engineers to develop environmen- controlled consumption of the cathode material while mini- tally responsive technology to revolutionize chemical pro- mizing the simultaneous formation of hydrogen. In this cesses for the next century. manner, arsine yields ofnearly 85% are obtained in H2 by the reactions 1. Landau, R. & Rosenberg, N. (1990) Invent. Technol., 58-63. 2. Heafon, G., Repetto, R. & Sobin, R. (1991) Transforming As(s) + 3H20 + 3e -* + 30H- Technology: An Agenda for Environmentally Sustainable AsH3(,) Growth in the 21st Century (World Resource Institute, Wash- 2H20 + 2e- H2g) + 20H- ington), p. 5. 3. Medvedev, G. (1991) The Truth About Chernobyl (Basic Books, Other components of the generator include two drying col- New York). umns in tandem and automated pressure and flow regulation 4. Dombrowski, S. L. S. (April 1991) Environ. Protect. 2, 14-22. controls. The entire system is controlled by a microprocessor 5. News Updates (1991) Environ. Protect. 2, 8. 6. McGregor, G. (1991) Environ. Protect. 2, 25, 54. and a direct-current power supply. This system is designed to 7. Lepkowski, W. (1991) Chem. Eng. News 69, 13-16. be housed completely within a standard toxic-gas cabinet. 8. Relsch, M. S. (1991) Chem. Eng. News 69, 13-16. AT&T has licensed Electron Transfer Technologies (Princ- 9. Sillmon, R. S. & Freitas, J. (1990) Appl. Phys. Lett. 56, eton, NJ) to supply the system commercially. 174-176. The high quality of generated arsine is established by 10. Lum, R. M., Klingert, J. K. & Kisker, D. W. (1989) J. Appl. detailed chemical characterizations. Arsine and hydrogen Phys. 66, 652-655. levels measured by on-line acoustic time-of-flight techniques 11. Lee, P. W., Omstead, T. R., McKenna, D. R. & Jensen, K. F. are given in Table 7. The absence of impurities of concern to (1988) J. Cryst. Growth 93, 134-142. on-line mass 12. Valdes, J. L., Cadet, G. & Mitchell, J. W. (1991) J. Electro- device growers is assured by spectrometry. chem. Soc., in press. Moisture is also measured specifically by a high-sensitivity 13. Lum, R. M., Klingert, J. K., Ren, F. & Shah, N. J. (1990) sensor. The moisture level, 80 + 2 ppb, is more than an order Appl. Phys. Lett. 56, 379-381. of magnitude lower than the best purity quoted by commer- 14. Buckley D. N., Seabury, C. W., Valtes, J. L., Cadet, G., cial suppliers of arsine in cylinders. Oxygen impurities have Mitchell, J. W., DiGiuseppe, M. A., Smith, R. C., Filipes not been detected by any of the applicable analytical meth- J. R. C., Bylsma, R. B., Chakrabarti, U. K. & Wang, K.-W. ods. During periods when the generator is in idle mode, (1990) Appl. Phys. Lett. 57, 177-182. Downloaded by guest on September 29, 2021