- Email: [email protected]
A quantum leap in the development of new materials and devices
Applied Surface Science 223 (2004) 1–8
A quantum leap in the development of new materials and devices Joseph J. Hanak* 4125 Aplin Road, Ames, IA 50014, USA
Abstract A review is given of the ‘‘Multiple Sample Concept’’ (MSC), developed at RCA Laboratories in the 1970s, that has led to a rapid increase in the development of new materials and devices. MSC consists of synthesis, analysis, testing, and evaluation of entire multi-component systems. Examples are given of the use of MSC in developing new materials and devices for a variety of electronic applications. # 2003 Elsevier B.V. All rights reserved. Keywords: Combinatorial; Cermets; Superconductors; Ferromagnets; Electroluminescence; (a-Si:H) PV cells
1. Introduction The traditional approach to the search for new materials has been that of handling one material at a time in the processes of synthesis, chemical analysis and testing of properties, a time-consuming and expensive process. There were several reasons for searching for a faster process, the main one being that the number of known materials in comparison to the possible ones is dismally small for the combinations of three or more elements and that the situation becomes progressively worse for higher combinations. Consequently, in the early days of my career, I have sought a way of increasing the productivity of the search for new materials and have succeeded in introducing a novel approach named the ‘‘Multiple Sample Concept’’ (MSC) [1–3]. The present paper reviews the concepts involved and lists some of the results and useful consequences of this approach. MSC has been * Tel.: þ1-515-292-8251. E-mail address: [email protected] (J.J. Hanak).
identified by the Chemical and Engineering News [4] and two other publications as the forerunner of the ‘‘Combinatorial Materials Science’’.
2. Outline of the multiple sample concept The automated approach involves processing of entire or partial multi-component system at one time, instead of just one composition. A significant increase in the rate of acquiring new information results from the fact that a given multi-component system can be processed in almost the same time as a single sample. Implementation of the new approach involved first adopting a radio-frequency (rf) co-sputtering technique, capable of synthesizing most and nearly complete binary and/or ternary solid alloy systems, regardless of their miscibility. Figs. 1 and 2 show a schematic arrangement of the targets and substrates for co-sputtering films of two-component systems. Three-component systems were sputtered using targets having three segments.
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00902-4
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J.J. Hanak / Applied Surface Science 223 (2004) 1–8
Fig. 1. Schematic arrangement for co-sputtering two-component systems (from [1]).
The second step in implementing the MSC approach was the development of a unique method of compositional determination applicable to all cosputtered films. The sputtering arrangement in Fig. 2 that uses polar coordinates is helpful in explaining this method [2]. On the target, point ‘‘O’’ is the center of the target, point ‘‘O0 ’’ is a reference point on the substrate (here, above the center of the target disk), and PO is any point on the substrate. This method is based on the assumption of a superposition principle which states that the total film thickness T(PO) at any point PO on the substrate consists of the sum n of the thicknesses, Tj(PO) of each constituent, j. n n X X (1) TðPO Þ ¼ t Tj ðPO Þ ¼ t Gj ðPO ÞRj ðO0 Þ j¼1
Fig. 2. Sputtering arrangement showing polar coordinates appropriate for segment of annulus and disk segment (from [2]).
j¼1
Eq. (1) also shows that each thickness consists of a product of the sputtering time, t, a deposition profile, Gj(PO), which is assumed to be dependent only on the geometry of the sputtering arrangement, and a deposition rate, Rj(O0 ), measured at some reference point on the substrate. In the right-hand side of Eq. (1), t is known and Gj(PO) can be determined empirically [1] or calculated from expressions derived [2] from Knudsen’s Cosine Law. Expressions have been derived for a
J.J. Hanak / Applied Surface Science 223 (2004) 1–8
3
Fig. 3. Experimental and theoretical profiles for a 1208 disk segment (from [2]).
segment of an annulus, and for a rectangle, which are applicable to calculating deposition profiles of most practical target shapes. This method became very useful for calculating the deposition profiles for composite targets. An example of a deposition profile for a ternary target having a shape of a 1208 segment of a disk is shown in Fig. 3 [2]. The unknown deposition rates Rj(O0 ) can be determined in several ways. One of these is by measuring the total thickness T(PO) at n or more locations on the substrate, substituting them in Eq. (1) and solving the resulting simultaneous equations [1]. For low concentrations, such as doping, one method consists of measuring the target emission rates by weight loss or sputter-etch depths and the sticking coefficients and then calculating Rj(O0 ) [3]. Another method consists of using actual chemical or instrumental analysis and thickness measurement on one or more points and calculating Rj(O0 ) [3]. Once the rates Rj(O0 ) are known, all thicknesses can be calculated at any point of the substrate as implied
by Eq. (1). Composition in volume percent can be obtained from Eq. (2): Tj ðPO Þ (2) vol:%j ¼ 100 TðPO Þ Composition in terms of mol% can be obtained with the knowledge of density and atomic weight of each of the components. Although approximate, the first method of determining Rj(O0 ), which is applicable to concentrations of 15 to 85%, has been shown to yield accuracy (RMS) of better than 10% of the amount of each constituent [2]. The second method, applicable from large concentrations down to about 0.1%, yielded accuracy of 25% or better. The third method, remaining to be tested, should be applicable to concentrations in the ppm level. The third step of implementing the MSC involved the development of apparatus and methods, which facilitated measurement of materials’ properties by scanning methods [5–7]. The final step involved
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J.J. Hanak / Applied Surface Science 223 (2004) 1–8
collection of the large amounts of data by means of data acquisition system and by computer processing, both of which were in their infancy. Although the reliability of co-sputtering and the method of compositional calculations have withstood numerous tests, there have been reports of unexpected phenomena, which posed serious challenges. Among them are target cross-contamination, substrate bombardment by focused secondary electrons and (then) newly observed etching of the substrate by anions [8]. Assessment of these problems on MSC appears in [9].
3. Electronic materials studies The MSC process has been applied to the study of electronic materials of interest to RCA. They included materials with superconducting, magnetic, electrical, optical, electro-luminescent, photoconducting and
photovoltaic properties. For binary systems, deposited from two half-disk targets, the films were deposited onto 12.5-cm-long, glass or ceramic, alumina substrates. The substrates were pre-coated with fifty narrow gold strips to serve as electrical contacts for the sputtered layers. For ternary systems, square, flat substrates were used. A photograph of the rear side of test sample having continuously variable parameters over the surface is shown in Fig. 4 [19]. A square matrix of metal electrodes is deposited for materials requiring electrical measurements such as PV cells and electroluminescent cells. Tested variables, dependent of the continuously built-in twin sets of variables, included photo-voltage (Voc), photocurrent (Jsc), fill factor (FF) cell efficiency (Z), optical stability and electroluminescence. One example of data for a ternary system appears in Fig. 6. Several of these materials were cermets, which are mixtures of finely divided metal particles and dielec-
Fig. 4. Rear view of a solar cell sample having graded variables in the x- and y- directions and a matrix of 25 25 metal electrodes to yield sets of performance data (from [19]).
J.J. Hanak / Applied Surface Science 223 (2004) 1–8
tric particles, formed preferably by rf co-sputtering. When the volume fraction of the metal component is large, the metallic particles touch and the film exhibits metallic behavior. As the volume fraction of the metal decreases below approximately 0.6, the metal particles become smaller and isolated and the properties become non-metallic. Electrical resistivity increases rapidly with decreasing metal content, exhibiting a negative temperature coefficient. The ordering temperature for both ferromagnetism and superconductivity decreases rapidly, and the films become transparent in the infrared. 3.1. Granular and compound superconductors Binary mixtures of several metals and silica were co-sputtered in the search for high superconducting (sc) transition temperature (Tc) [6,7]. Several metals have shown an increase in Tc, as for example Mo-SiO2, for which Tc increased from 0.9 to 6.3 K. while others, such as lanthanum, have shown a decrease in Tc. Mixtures of immiscible metals also show typical granular metal behavior, as shown in Fig. 5. When
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both metals show enhancement in Tc of a cermet, a maximum in Tc occurs; when one metal enhances and the other does not, a maximum and a minimum in Tc are observed. Finally, when one metal enhances and the other is not a superconductor, a maximum in Tc is observed and the curve extrapolates to zero at some intermediate composition. In the category of the A15 compounds, work has been done [10] both to reproduce Tc values of bulk materials in films and also to obtain metastable compositions with Tc higher than obtainable in the bulk. For example, the compound Nb3Ge, when prepared by sintering, is deficient in Ge and its Tc is low (6 K), while when co-sputtered onto frigid alumina substrates, followed by careful anneal, its Tc increases up to 17 K. 3.2. Granular ferromagnets Following the analogy of granular superconductors, Gittleman et al. [11] carried out a systematic study of the electrical and magnetic properties of co-sputtered Ni-SiO2 system as a function of composition, including
Fig. 5. Superconducting transition temperature vs. composition for binary granular film mixtures of immiscible metals (from [25]).
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Fig. 6. Contours of initial permeability as a function of composition for the ternary (NiyFe1y)1x(SiO2)x system (from [7]).
resistivity, magneto-resistance and magnetization, the latter by the use of the magneto-optic Kerr effect. Rayl et al. [12], also at RCA, have shown a decrease in Ni grain size and in Curie temperature with decreasing Ni content for similar samples. Noting a wide range of permeabilities available in the Fe-SiO2 and 80 permalloy-SiO2 cermets, Hanak and Gittleman [7] cosputtered the ternary system (NiyFe1y)1x(SiO2)x and measured its permeability, shown in Fig. 6, and electrical resistivity (not shown). The data indicated that the material (Ni0.7Fe0.3)0.55(SiO2)0.45 (by volume) had a particularly prominent permeability peak of m ’ 170 at a rather high resistivity of 101 ohm-cm. Such material is potentially interesting for high-frequency applications, such as magnetic recording heads or inductor cores. This composition was reproduced in thick continuous films by using either sintered powder targets or a disk target of Fe overlayed with islands of Ni and SiO2.
3.3. Optical properties of cermets The first systematic investigation of the optical properties of co-sputtered Ag-SiO2 and Au-SiO2 films, varied over nearly the entire range, has been made by Cohen et al. [13], also at RCA. They have observed the gradual evolution of the characteristic absorption peaks in the visible and have studied the transition from metallic to dielectric behavior in the near-infrared. The results have been interpreted in terms of the theory of Maxwell–Garnett, which takes into account the modification of the applied electric field at any point within the medium by the dipole fields of the surrounding metal particles. One practical application of an optical property of cermets (e.g., Ni-SiO2) involved their use in developing an improved intensity-correction filter for adjusting the light intensity over the field of the cathode ray tube (CRT) [14].
J.J. Hanak / Applied Surface Science 223 (2004) 1–8
3.4. Electroluminescence The system ZnS:Mnx:Cuy is well known for its yellow dc EL. However, EL cells that reached efficiency and life practical for displays had been made only from powder phosphors. In this work [15], a thorough study has been made of the dc EL properties of this system on co-sputtered films on glass and sandwiched between transparent, conducting SnO2 or In2O3 and a currentlimiting, co-sputtered layer of Ni-SiO2 cermet. A square array of EL cells was then formed and the EL performance tested. The optimum dopant concentration, on the basis of this sample, was Cu 0:3% and Mn 0:7%. Mean brightness of up to 770 and 20 fL with pulsed dc excitation at 12.5 and 0.1% duty cycle, respectively, were achieved, and a bright clock display was made. Additional systems were successfully studied to determine their potential applications for EL TV screens. They included green-emitting ZnS:Tb3þ and a new, red-emitting ternary, all oxide EL film [(Y2O3)y1 (In2O3)y]1x:(Eu2O3)x [16,17]. 3.5. Photovoltaics The development of hydrogenated amorphous silicon (a-Si:H) photovoltaic (PV) cells was first initiated at RCA by Carlson and Wronski [18]. The cells are multi-layer, thin-film devices, which were subject to the optimization of numerous of its parameters before becoming applicable for commercial use. They include: the deposition temperature, thickness, composition, deposition rate, gas pressure, deposition methods, deposition geometry, cell structures, front and rear contact layers, and others. In our approach to optimization [19,20], we used the MSC to generate square, planar samples, shown in Fig. 4 [19], having continuously graded pairs of such parameters at right angles to each other along the surface [2–4]. Co-sputtered layers of a variety of cermets were used as the contact layers to enhance the open-circuit voltage of the cells. The a-Si:H films have been deposited by decomposition of silane (SiH4), using the rf-capacitive, glowdischarge technique. The cell structure consisted of a glass substrate coated with transparent, conducting layer of In2O3, thin cermet layer (Pt-Y2O3), p-i-n layers of a-Si:H, and a film of rear metal electrode (Cr/Al).
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Pure SiH4 was used to form undoped i-layers. Mixtures of PH3 or B2H6 in hydrogen and silane to form n- and p-doped layers of a-Si:H. Of particular interest for this paper was the method of deposition of the semiconducting films having continuous variation of the substrate deposition temperature (T) and film thickness (D) to be applicable to MSC. Variation of T was accomplished by mounting the glass substrate, in close contact, onto the bottom side of a graphite plate, heated by a cylindrical quartz lamp inserted in a cylindrical hole on one side of the plate. Typically, a gradient of up to a 220 8C was generated along the plate. Variation of film thickness D along the transverse direction was achieved by using a motordriven square mask covering the substrate surface. Tested variables, dependent of the continuously builtin twin variables, included photo-voltage (Voc), photocurrent (Jsc), fill factor (FF) cell efficiency (Z), and optical stability. Steady improvements of the performance of a-Si:H PV cells have been achieved, which then called for the development of series-interconnected cells to form simplified PV panels having desired voltage to power devices. A ‘‘Monolithic Solar Cell Panel of Amorphous Silicon’’ was developed [21], starting with a large, continuous cell, then separated by laser scribing in one direction into parallel, 0.7-cm wide cells, and then interconnected by sputtered or evaporated metal film, assisted by further laser scribing. This method was successful and remains in use in commercial PV arrays to date. Another type of device that had been developed is that of multiple-junction or ‘‘stacked’’ solar cells because this concept held promise of improved efficiency and stability [22,23]. The stacked cell thickness design was as follows. The front cell had its i-layer graded from 20 to 350 nm, while for the second cell it was graded in transverse direction from 120 to 800 nm. The optimum cell performance occurred at i-layer thickness of 38 nm for the first cell and 600 nm for the second cell. Later, a-SixGe(1x)H were used as the second or third cells in the stacked cells to increase the efficiency.
4. Conclusions At RCA, MSC has been applied successfully in generating 12 U.S. Patents in the areas of cermets
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including CRT applications, superconductors, magnetic recording, and for solar cell contact films; EL films emitting yellow, green and red light, liquid crystal devices having diode characteristics; and a variety of photovoltaic cell applications. The technology that benefited most from MSC was thin-film a-Si:H photovoltaics, in which RCA made a significant progress. Nevertheless, after making estimates of costs of its commercialization, RCA sold the technology to Solarex, now owned by BP Solar, which is the largest a-Si:H PV array manufacturer. The next significant manufacturer of a-Si:H PV arrays in Troy, MI, is Baekart-USSC, which produces flexible arrays. Various other companies in USA and worldwide are also producing a-Si:H arrays. The technology that was likely to benefit RCA from MSC was superconductivity. RCA pioneered the production of high-field sc magnets using sc metal ribbons coated by a chemical vapor deposition (CVD) process of Nb3Sn [24]. In search for much better materials, up to nine times greater increases in Tc in binary granular metal mixtures have been discovered using MSC. However, in 1970, RCA sold the sc technology to a General Electric (GE) spinoff, Intermagnetics General (IG). Since then, the high-Tc and high-sc magnet technology has progressed immensely, with IG as one of the leading companies, partially based on RCA’s innovations. Later, in 1985, the entire RCA was sold to GE. Acknowledgements Acknowledgements are due to personnel at RCA David Sarnoff Research Center, in Princeton, NJ, including Fred Rosi, a V. P. of happy memory, George Cody and David Carlson, group leaders in superconductivity and photovoltaics and numerous collaborators, most of whom are listed as co-authors in the References. Special acknowledgements are due to Hans W. Lehmann and Roland K. Wehner [3], of former RCA Labs in Zurich, Switzerland, for their contribution in developing the method of calculating the deposition profiles based on the Knudsen’s Cosine Law.
References [1] J.J. Hanak, J. Mater. Sci. 5 (1970) 964–971. Chapman and Hall Ltd. [2] J.J. Hanak, H.W. Lehmann, R.K. Wehner, J. Appl. Phys. 43 (1972) 1666. [3] J.J. Hanak, B.F.K. Bolker, J. Appl. Phys. 44 (11) (1973) 5142. [4] Ron Dagani, Chemical and Engineering News 77, (10), 51, March 8 (1999). [5] J.J. Hanak, J.I. Gittleman, J.P. Pellicane, S. Bozowski, Phys. Lett. 30A (1969) 201. [6] J.J. Hanak, J.I. Gittleman, Physica 55 (1971) 555. [7] J.J. Hanak, J.I. Gittleman, Magnetism and Magnetic Materials – 1972, in: Graham and Rhyne (Eds.), AIP Conference Proceedings Part 2, (10) New York, 1973, p. 961. [8] J.J. Hanak, J.P. Pellicane, J. Vac. Sci. Technol. 13 (2) (1976). [9] J.J. Hanak, Le Vide, No. 175 – Janvier-Fevrier (1975). [10] J.J. Hanak, J.I. Gittleman, J.P. Pellicane, S. Bozowski, J. Appl. Phys. 41 (1970) 4958. [11] J.I. Gittleman, Y. Goldstein, S. Bozowski, Phys. Rev. B1 (5) (1972) 3609. [12] M. Rayl, P.J. Wojtowicz, M.S. Abrahams, R.L. Harvey, C.J. Buiocchi, Magnetism and Magnetic Materials – 1972, in: Graham and Rhyne (Eds.), AIP Conference Proceedings Part 2, (5) New York, 1972, p. 472. [13] R.W. Cohen, G.D. Cody, M.D. Coutts, B. Abeles, Phys. Rev. B8 (8) (1973) 3689. [14] J.J. Hanak, U.S. Patent No. 4157215, Photodeposition of CRT Screen Structures Using Cermet IC Filter, June 5, 1979. [15] J.J. Hanak, Jpn. J. Appl. Phys. (Suppl. 2) (Part 1) (1974) 809. [16] J.I. Pankove, M.A. Lampert, J.J. Hanak, J.E. Berkeyheiser, J. Lumin. 15 (1977) 349. [17] J.J. Hanak, US Patent No. 4027192, Electroluminescent Device Comprising Electroluminescent Layer of Indium Oxide and/or Tin Oxide, 31 May 1975. [18] D.E. Carlson, C.R. Wronski, Appl. Phys. Lett. 28 (11) (1976) 671. [19] J.J. Hanak, V. Korsun, J.P. Pellicane, in: Proceedings of the Second European Communities Photovoltaic Solar Energy Conference, Berlin (West), 1979. [20] J.J. Hanak, V. Korsun, J.P. Pellicane, in: Proceedings of the 15th IEEE Photovoltaic Specialists Conference, 1981, p. 697. [21] J.J. Hanak, B. Faughnan, V. Korsun, J.P. Pellicane, in: Proceedings of the 14th EEEE Photovoltaic Specialists Conference, San Diego, CA, January 1980, pp. 1209–1213. [22] J.J. Hanak, J. Non-Cryst. Solids 35–36 (1980) 755. [23] J.J. Hanak, Solar Energy 23 (1979) 145. [24] Synthesis, Characterization, and Application of Superconducting Niobium Stannide Nb3Sn, RCA Review XXV, (3), Special Issue, September 1964. [25] J.J. Hanak, Suppl. to No. 165 of the Review Le Vide, Les Couches Minces, 1973, p. 177.
A quantum leap in the development of new materials and devices Joseph J. Hanak* 4125 Aplin Road, Ames, IA 50014, USA
Abstract A review is given of the ‘‘Multiple Sample Concept’’ (MSC), developed at RCA Laboratories in the 1970s, that has led to a rapid increase in the development of new materials and devices. MSC consists of synthesis, analysis, testing, and evaluation of entire multi-component systems. Examples are given of the use of MSC in developing new materials and devices for a variety of electronic applications. # 2003 Elsevier B.V. All rights reserved. Keywords: Combinatorial; Cermets; Superconductors; Ferromagnets; Electroluminescence; (a-Si:H) PV cells
1. Introduction The traditional approach to the search for new materials has been that of handling one material at a time in the processes of synthesis, chemical analysis and testing of properties, a time-consuming and expensive process. There were several reasons for searching for a faster process, the main one being that the number of known materials in comparison to the possible ones is dismally small for the combinations of three or more elements and that the situation becomes progressively worse for higher combinations. Consequently, in the early days of my career, I have sought a way of increasing the productivity of the search for new materials and have succeeded in introducing a novel approach named the ‘‘Multiple Sample Concept’’ (MSC) [1–3]. The present paper reviews the concepts involved and lists some of the results and useful consequences of this approach. MSC has been * Tel.: þ1-515-292-8251. E-mail address: [email protected] (J.J. Hanak).
identified by the Chemical and Engineering News [4] and two other publications as the forerunner of the ‘‘Combinatorial Materials Science’’.
2. Outline of the multiple sample concept The automated approach involves processing of entire or partial multi-component system at one time, instead of just one composition. A significant increase in the rate of acquiring new information results from the fact that a given multi-component system can be processed in almost the same time as a single sample. Implementation of the new approach involved first adopting a radio-frequency (rf) co-sputtering technique, capable of synthesizing most and nearly complete binary and/or ternary solid alloy systems, regardless of their miscibility. Figs. 1 and 2 show a schematic arrangement of the targets and substrates for co-sputtering films of two-component systems. Three-component systems were sputtered using targets having three segments.
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00902-4
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J.J. Hanak / Applied Surface Science 223 (2004) 1–8
Fig. 1. Schematic arrangement for co-sputtering two-component systems (from [1]).
The second step in implementing the MSC approach was the development of a unique method of compositional determination applicable to all cosputtered films. The sputtering arrangement in Fig. 2 that uses polar coordinates is helpful in explaining this method [2]. On the target, point ‘‘O’’ is the center of the target, point ‘‘O0 ’’ is a reference point on the substrate (here, above the center of the target disk), and PO is any point on the substrate. This method is based on the assumption of a superposition principle which states that the total film thickness T(PO) at any point PO on the substrate consists of the sum n of the thicknesses, Tj(PO) of each constituent, j. n n X X (1) TðPO Þ ¼ t Tj ðPO Þ ¼ t Gj ðPO ÞRj ðO0 Þ j¼1
Fig. 2. Sputtering arrangement showing polar coordinates appropriate for segment of annulus and disk segment (from [2]).
j¼1
Eq. (1) also shows that each thickness consists of a product of the sputtering time, t, a deposition profile, Gj(PO), which is assumed to be dependent only on the geometry of the sputtering arrangement, and a deposition rate, Rj(O0 ), measured at some reference point on the substrate. In the right-hand side of Eq. (1), t is known and Gj(PO) can be determined empirically [1] or calculated from expressions derived [2] from Knudsen’s Cosine Law. Expressions have been derived for a
J.J. Hanak / Applied Surface Science 223 (2004) 1–8
3
Fig. 3. Experimental and theoretical profiles for a 1208 disk segment (from [2]).
segment of an annulus, and for a rectangle, which are applicable to calculating deposition profiles of most practical target shapes. This method became very useful for calculating the deposition profiles for composite targets. An example of a deposition profile for a ternary target having a shape of a 1208 segment of a disk is shown in Fig. 3 [2]. The unknown deposition rates Rj(O0 ) can be determined in several ways. One of these is by measuring the total thickness T(PO) at n or more locations on the substrate, substituting them in Eq. (1) and solving the resulting simultaneous equations [1]. For low concentrations, such as doping, one method consists of measuring the target emission rates by weight loss or sputter-etch depths and the sticking coefficients and then calculating Rj(O0 ) [3]. Another method consists of using actual chemical or instrumental analysis and thickness measurement on one or more points and calculating Rj(O0 ) [3]. Once the rates Rj(O0 ) are known, all thicknesses can be calculated at any point of the substrate as implied
by Eq. (1). Composition in volume percent can be obtained from Eq. (2): Tj ðPO Þ (2) vol:%j ¼ 100 TðPO Þ Composition in terms of mol% can be obtained with the knowledge of density and atomic weight of each of the components. Although approximate, the first method of determining Rj(O0 ), which is applicable to concentrations of 15 to 85%, has been shown to yield accuracy (RMS) of better than 10% of the amount of each constituent [2]. The second method, applicable from large concentrations down to about 0.1%, yielded accuracy of 25% or better. The third method, remaining to be tested, should be applicable to concentrations in the ppm level. The third step of implementing the MSC involved the development of apparatus and methods, which facilitated measurement of materials’ properties by scanning methods [5–7]. The final step involved
4
J.J. Hanak / Applied Surface Science 223 (2004) 1–8
collection of the large amounts of data by means of data acquisition system and by computer processing, both of which were in their infancy. Although the reliability of co-sputtering and the method of compositional calculations have withstood numerous tests, there have been reports of unexpected phenomena, which posed serious challenges. Among them are target cross-contamination, substrate bombardment by focused secondary electrons and (then) newly observed etching of the substrate by anions [8]. Assessment of these problems on MSC appears in [9].
3. Electronic materials studies The MSC process has been applied to the study of electronic materials of interest to RCA. They included materials with superconducting, magnetic, electrical, optical, electro-luminescent, photoconducting and
photovoltaic properties. For binary systems, deposited from two half-disk targets, the films were deposited onto 12.5-cm-long, glass or ceramic, alumina substrates. The substrates were pre-coated with fifty narrow gold strips to serve as electrical contacts for the sputtered layers. For ternary systems, square, flat substrates were used. A photograph of the rear side of test sample having continuously variable parameters over the surface is shown in Fig. 4 [19]. A square matrix of metal electrodes is deposited for materials requiring electrical measurements such as PV cells and electroluminescent cells. Tested variables, dependent of the continuously built-in twin sets of variables, included photo-voltage (Voc), photocurrent (Jsc), fill factor (FF) cell efficiency (Z), optical stability and electroluminescence. One example of data for a ternary system appears in Fig. 6. Several of these materials were cermets, which are mixtures of finely divided metal particles and dielec-
Fig. 4. Rear view of a solar cell sample having graded variables in the x- and y- directions and a matrix of 25 25 metal electrodes to yield sets of performance data (from [19]).
J.J. Hanak / Applied Surface Science 223 (2004) 1–8
tric particles, formed preferably by rf co-sputtering. When the volume fraction of the metal component is large, the metallic particles touch and the film exhibits metallic behavior. As the volume fraction of the metal decreases below approximately 0.6, the metal particles become smaller and isolated and the properties become non-metallic. Electrical resistivity increases rapidly with decreasing metal content, exhibiting a negative temperature coefficient. The ordering temperature for both ferromagnetism and superconductivity decreases rapidly, and the films become transparent in the infrared. 3.1. Granular and compound superconductors Binary mixtures of several metals and silica were co-sputtered in the search for high superconducting (sc) transition temperature (Tc) [6,7]. Several metals have shown an increase in Tc, as for example Mo-SiO2, for which Tc increased from 0.9 to 6.3 K. while others, such as lanthanum, have shown a decrease in Tc. Mixtures of immiscible metals also show typical granular metal behavior, as shown in Fig. 5. When
5
both metals show enhancement in Tc of a cermet, a maximum in Tc occurs; when one metal enhances and the other does not, a maximum and a minimum in Tc are observed. Finally, when one metal enhances and the other is not a superconductor, a maximum in Tc is observed and the curve extrapolates to zero at some intermediate composition. In the category of the A15 compounds, work has been done [10] both to reproduce Tc values of bulk materials in films and also to obtain metastable compositions with Tc higher than obtainable in the bulk. For example, the compound Nb3Ge, when prepared by sintering, is deficient in Ge and its Tc is low (6 K), while when co-sputtered onto frigid alumina substrates, followed by careful anneal, its Tc increases up to 17 K. 3.2. Granular ferromagnets Following the analogy of granular superconductors, Gittleman et al. [11] carried out a systematic study of the electrical and magnetic properties of co-sputtered Ni-SiO2 system as a function of composition, including
Fig. 5. Superconducting transition temperature vs. composition for binary granular film mixtures of immiscible metals (from [25]).
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Fig. 6. Contours of initial permeability as a function of composition for the ternary (NiyFe1y)1x(SiO2)x system (from [7]).
resistivity, magneto-resistance and magnetization, the latter by the use of the magneto-optic Kerr effect. Rayl et al. [12], also at RCA, have shown a decrease in Ni grain size and in Curie temperature with decreasing Ni content for similar samples. Noting a wide range of permeabilities available in the Fe-SiO2 and 80 permalloy-SiO2 cermets, Hanak and Gittleman [7] cosputtered the ternary system (NiyFe1y)1x(SiO2)x and measured its permeability, shown in Fig. 6, and electrical resistivity (not shown). The data indicated that the material (Ni0.7Fe0.3)0.55(SiO2)0.45 (by volume) had a particularly prominent permeability peak of m ’ 170 at a rather high resistivity of 101 ohm-cm. Such material is potentially interesting for high-frequency applications, such as magnetic recording heads or inductor cores. This composition was reproduced in thick continuous films by using either sintered powder targets or a disk target of Fe overlayed with islands of Ni and SiO2.
3.3. Optical properties of cermets The first systematic investigation of the optical properties of co-sputtered Ag-SiO2 and Au-SiO2 films, varied over nearly the entire range, has been made by Cohen et al. [13], also at RCA. They have observed the gradual evolution of the characteristic absorption peaks in the visible and have studied the transition from metallic to dielectric behavior in the near-infrared. The results have been interpreted in terms of the theory of Maxwell–Garnett, which takes into account the modification of the applied electric field at any point within the medium by the dipole fields of the surrounding metal particles. One practical application of an optical property of cermets (e.g., Ni-SiO2) involved their use in developing an improved intensity-correction filter for adjusting the light intensity over the field of the cathode ray tube (CRT) [14].
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3.4. Electroluminescence The system ZnS:Mnx:Cuy is well known for its yellow dc EL. However, EL cells that reached efficiency and life practical for displays had been made only from powder phosphors. In this work [15], a thorough study has been made of the dc EL properties of this system on co-sputtered films on glass and sandwiched between transparent, conducting SnO2 or In2O3 and a currentlimiting, co-sputtered layer of Ni-SiO2 cermet. A square array of EL cells was then formed and the EL performance tested. The optimum dopant concentration, on the basis of this sample, was Cu 0:3% and Mn 0:7%. Mean brightness of up to 770 and 20 fL with pulsed dc excitation at 12.5 and 0.1% duty cycle, respectively, were achieved, and a bright clock display was made. Additional systems were successfully studied to determine their potential applications for EL TV screens. They included green-emitting ZnS:Tb3þ and a new, red-emitting ternary, all oxide EL film [(Y2O3)y1 (In2O3)y]1x:(Eu2O3)x [16,17]. 3.5. Photovoltaics The development of hydrogenated amorphous silicon (a-Si:H) photovoltaic (PV) cells was first initiated at RCA by Carlson and Wronski [18]. The cells are multi-layer, thin-film devices, which were subject to the optimization of numerous of its parameters before becoming applicable for commercial use. They include: the deposition temperature, thickness, composition, deposition rate, gas pressure, deposition methods, deposition geometry, cell structures, front and rear contact layers, and others. In our approach to optimization [19,20], we used the MSC to generate square, planar samples, shown in Fig. 4 [19], having continuously graded pairs of such parameters at right angles to each other along the surface [2–4]. Co-sputtered layers of a variety of cermets were used as the contact layers to enhance the open-circuit voltage of the cells. The a-Si:H films have been deposited by decomposition of silane (SiH4), using the rf-capacitive, glowdischarge technique. The cell structure consisted of a glass substrate coated with transparent, conducting layer of In2O3, thin cermet layer (Pt-Y2O3), p-i-n layers of a-Si:H, and a film of rear metal electrode (Cr/Al).
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Pure SiH4 was used to form undoped i-layers. Mixtures of PH3 or B2H6 in hydrogen and silane to form n- and p-doped layers of a-Si:H. Of particular interest for this paper was the method of deposition of the semiconducting films having continuous variation of the substrate deposition temperature (T) and film thickness (D) to be applicable to MSC. Variation of T was accomplished by mounting the glass substrate, in close contact, onto the bottom side of a graphite plate, heated by a cylindrical quartz lamp inserted in a cylindrical hole on one side of the plate. Typically, a gradient of up to a 220 8C was generated along the plate. Variation of film thickness D along the transverse direction was achieved by using a motordriven square mask covering the substrate surface. Tested variables, dependent of the continuously builtin twin variables, included photo-voltage (Voc), photocurrent (Jsc), fill factor (FF) cell efficiency (Z), and optical stability. Steady improvements of the performance of a-Si:H PV cells have been achieved, which then called for the development of series-interconnected cells to form simplified PV panels having desired voltage to power devices. A ‘‘Monolithic Solar Cell Panel of Amorphous Silicon’’ was developed [21], starting with a large, continuous cell, then separated by laser scribing in one direction into parallel, 0.7-cm wide cells, and then interconnected by sputtered or evaporated metal film, assisted by further laser scribing. This method was successful and remains in use in commercial PV arrays to date. Another type of device that had been developed is that of multiple-junction or ‘‘stacked’’ solar cells because this concept held promise of improved efficiency and stability [22,23]. The stacked cell thickness design was as follows. The front cell had its i-layer graded from 20 to 350 nm, while for the second cell it was graded in transverse direction from 120 to 800 nm. The optimum cell performance occurred at i-layer thickness of 38 nm for the first cell and 600 nm for the second cell. Later, a-SixGe(1x)H were used as the second or third cells in the stacked cells to increase the efficiency.
4. Conclusions At RCA, MSC has been applied successfully in generating 12 U.S. Patents in the areas of cermets
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including CRT applications, superconductors, magnetic recording, and for solar cell contact films; EL films emitting yellow, green and red light, liquid crystal devices having diode characteristics; and a variety of photovoltaic cell applications. The technology that benefited most from MSC was thin-film a-Si:H photovoltaics, in which RCA made a significant progress. Nevertheless, after making estimates of costs of its commercialization, RCA sold the technology to Solarex, now owned by BP Solar, which is the largest a-Si:H PV array manufacturer. The next significant manufacturer of a-Si:H PV arrays in Troy, MI, is Baekart-USSC, which produces flexible arrays. Various other companies in USA and worldwide are also producing a-Si:H arrays. The technology that was likely to benefit RCA from MSC was superconductivity. RCA pioneered the production of high-field sc magnets using sc metal ribbons coated by a chemical vapor deposition (CVD) process of Nb3Sn [24]. In search for much better materials, up to nine times greater increases in Tc in binary granular metal mixtures have been discovered using MSC. However, in 1970, RCA sold the sc technology to a General Electric (GE) spinoff, Intermagnetics General (IG). Since then, the high-Tc and high-sc magnet technology has progressed immensely, with IG as one of the leading companies, partially based on RCA’s innovations. Later, in 1985, the entire RCA was sold to GE. Acknowledgements Acknowledgements are due to personnel at RCA David Sarnoff Research Center, in Princeton, NJ, including Fred Rosi, a V. P. of happy memory, George Cody and David Carlson, group leaders in superconductivity and photovoltaics and numerous collaborators, most of whom are listed as co-authors in the References. Special acknowledgements are due to Hans W. Lehmann and Roland K. Wehner [3], of former RCA Labs in Zurich, Switzerland, for their contribution in developing the method of calculating the deposition profiles based on the Knudsen’s Cosine Law.
References [1] J.J. Hanak, J. Mater. Sci. 5 (1970) 964–971. Chapman and Hall Ltd. [2] J.J. Hanak, H.W. Lehmann, R.K. Wehner, J. Appl. Phys. 43 (1972) 1666. [3] J.J. Hanak, B.F.K. Bolker, J. Appl. Phys. 44 (11) (1973) 5142. [4] Ron Dagani, Chemical and Engineering News 77, (10), 51, March 8 (1999). [5] J.J. Hanak, J.I. Gittleman, J.P. Pellicane, S. Bozowski, Phys. Lett. 30A (1969) 201. [6] J.J. Hanak, J.I. Gittleman, Physica 55 (1971) 555. [7] J.J. Hanak, J.I. Gittleman, Magnetism and Magnetic Materials – 1972, in: Graham and Rhyne (Eds.), AIP Conference Proceedings Part 2, (10) New York, 1973, p. 961. [8] J.J. Hanak, J.P. Pellicane, J. Vac. Sci. Technol. 13 (2) (1976). [9] J.J. Hanak, Le Vide, No. 175 – Janvier-Fevrier (1975). [10] J.J. Hanak, J.I. Gittleman, J.P. Pellicane, S. Bozowski, J. Appl. Phys. 41 (1970) 4958. [11] J.I. Gittleman, Y. Goldstein, S. Bozowski, Phys. Rev. B1 (5) (1972) 3609. [12] M. Rayl, P.J. Wojtowicz, M.S. Abrahams, R.L. Harvey, C.J. Buiocchi, Magnetism and Magnetic Materials – 1972, in: Graham and Rhyne (Eds.), AIP Conference Proceedings Part 2, (5) New York, 1972, p. 472. [13] R.W. Cohen, G.D. Cody, M.D. Coutts, B. Abeles, Phys. Rev. B8 (8) (1973) 3689. [14] J.J. Hanak, U.S. Patent No. 4157215, Photodeposition of CRT Screen Structures Using Cermet IC Filter, June 5, 1979. [15] J.J. Hanak, Jpn. J. Appl. Phys. (Suppl. 2) (Part 1) (1974) 809. [16] J.I. Pankove, M.A. Lampert, J.J. Hanak, J.E. Berkeyheiser, J. Lumin. 15 (1977) 349. [17] J.J. Hanak, US Patent No. 4027192, Electroluminescent Device Comprising Electroluminescent Layer of Indium Oxide and/or Tin Oxide, 31 May 1975. [18] D.E. Carlson, C.R. Wronski, Appl. Phys. Lett. 28 (11) (1976) 671. [19] J.J. Hanak, V. Korsun, J.P. Pellicane, in: Proceedings of the Second European Communities Photovoltaic Solar Energy Conference, Berlin (West), 1979. [20] J.J. Hanak, V. Korsun, J.P. Pellicane, in: Proceedings of the 15th IEEE Photovoltaic Specialists Conference, 1981, p. 697. [21] J.J. Hanak, B. Faughnan, V. Korsun, J.P. Pellicane, in: Proceedings of the 14th EEEE Photovoltaic Specialists Conference, San Diego, CA, January 1980, pp. 1209–1213. [22] J.J. Hanak, J. Non-Cryst. Solids 35–36 (1980) 755. [23] J.J. Hanak, Solar Energy 23 (1979) 145. [24] Synthesis, Characterization, and Application of Superconducting Niobium Stannide Nb3Sn, RCA Review XXV, (3), Special Issue, September 1964. [25] J.J. Hanak, Suppl. to No. 165 of the Review Le Vide, Les Couches Minces, 1973, p. 177.