The National Renewable Energy Laboratory (NREL) has extensive capabilities for fabricating a variety of high-technology films. Much of the in-house work in NREL's large photovoltaics (PV) program involves the fabrication of multiple thin-film semiconducting layers constituting a thin-film PV device. NREL's smaller program in superconductivity focuses on the fabrication of superconducting films on long, flexible tape substrates. This paper focuses on four of NREL's in-house research groups and their film fabrication techniques, developed for a variety of elements, alloys, and compounds to be deposited on a variety of substrates. As is the case for many national laboratories, NREL's technology transfer efforts are focusing on Cooperative Research and Development Agreements (CRADA's) between NREL researchers and private industry researchers

Mcconnell, Robert D.

English

NASA Technical Reports Server

Film Fabriwtion Technologies at NREL 
Robert D. McConnell 
National Renewable Energy Laboratory 
Golden, CO 80401 
ABSTRACT 
The National Renewable Energy Laboratory (NREL) has extensive capabilities for fabricating a vmety of 
high-technology films. Much of the in-house work in NREL's large photovoltaics (PV) program involves 
the fabrication of multiple thm-film semiconducting layers constituting a thin- film PV device. NIREVs 
smaller program in superconductivity focuses on the fabrication of superconducting films on long, flexnble- 
tape substrates. This paper focuses on four of NREL's in-house research groups and their film fabnca~on 
techniques, developed for a variety of elements, alloys, and compounds to be deposited on a variety of 
substrates. As is the case for many national laboratories, NREL's technology transfer efforts are Gocusmg 
on Cooperative Research and Development Agreements (CRADAs) between NREL researchers and prnvate 
industry researchers. The reader is encouraged to consider and explore the application of these film- 
fabrication technologies for their own needs by contacting the author or the principal scientists at NFEL, 
referenced below. 
INTRODUCTION 
Chemical deposition and physical deposition are the two principal classifications for a multihde of film- 
fabrication technologies, whether the films are thin (about 1 micron thick or less) or not (thcker &an 1 
micron). Among the chemical deposition technologies used by NREL researchers are several chemcal 
vapor deposition techniques, liquid phase epitaxy, solution growth, and several electrochemical kclasnnques. 
Among the physical deposition technologies are several sputtering techniques, physical vapor deposstim 
using electron beams or resistance heating, molecular beam epitaxy, and melt coating. These techniques or 
their variations have been used at NREL to deposit a variety of metals, compounds, and alloys as 
amorphous, polycrystalline, or crystalline films. Among the critical factors in identifying an appro~iak 
deposition technology are the control precision needed for the amounts of the elements used in the filn~s-- 
especially important in the case of semiconducting compounds and their dopants--the effect of the 
deposition technology on the crystallinity of the final films, and the deposition rate-an especially 
important parameter for production costing. 
A historical description of NREL's filrn fabrication technologies and the materials deposited is in reference 
1. The deposition techniques at NREL include electron beam evaporation, physical vapor deposition, 
magnetron (DC and RF) sputtering, ion-beam sputtering, electrodeposition of multielement compunds, 
molecular beam epitaxy, liquid-phase epitaxy, electrolytic plating, electroless deposition, plasma+&mcd 
chemical vapor deposition, photochemical vapor deposition, hot-wire catalytic deposition, melt sheet 
growth, and melt coating. Several of these techniques, or their variations, have been used to deposit films 
of Si, amorphous Si, amorphous Ge, amorphous C, Au, Sn, Zn, Ag, Mo, W, GaAs, GaAlAs, GAPz,  
GaAsP, InP, Gap, CuInSg, CdS, In203(Sn), Sn%(Sb), ZnO(Al), Ta205, Ti@, MgF2, ZnS, Mo03, 
WO3, diamond-like carbon films, YBaCuO compounds, BiSrCaCuO compounds, TlBaCaCuO compmds. 
other refractory metals, other metal oxides, silicon alloys, and amorphous-silicon alloys. 
NREL also has a long history of substantial funding (50% of NREL PV funds) for industrial research in 
photovoltaics; and much of that research is devoted to manufacturing processes, including filrn f a k c a ~ o n  
research for thin-film devices. Subcontract research to industry is a legitimate technology bzmsfer activity 
at NREL, although industry itself is supported to develop technology, and no "transfer" takes place. 
This paper will highlight four of NREL's film fabrication technologies and their achievements ~ & I !  
NREL's PV and superconductivity programs. Three are chemical deposition technologies that have proven 
to be advantageous for the controlled deposition of multielement compounds and alloys. The f o d  is a 
physical deposition technology with known advantages for use in production. NREL's in-house resmch 
capabilities are available to US PV companies, superconductivity companies, and other c o q a ~ e s  
interested in film-fabrication technologies. 
https://ntrs.nasa.gov/search.jsp?R=19930016374 2020-03-24T07:26:35+00:00Z
METALORGANIC CHEMICAL VAPOR DEPOSITION 
J. Olson and coworkers successfully fabricated record-high-efficiency photovoltaic devices using a custom- 
built metalorganic chemical-vapor-deposition system (2,3). They grew their epitaxial layers of GaInP.2 
me8 GaAs in an atmospheric-pressure MOCVD vertical reactor, and achieved efficiencies for the 
cmverslon of sunlight to electricity as high as 29% (active area), a record for this technology. The system 
was built with a standard run-vent configuration. The Ga, In, P, As, and Zn sources were compounds of 
tnmthylgalPium, trimethylindium, phosphine, arsine, and diethylzinc. Carbon doping came from a CC4 
s o m e  consisting of reagent-grade CC4  housed in a conventional bubbler, with Pd-diffused H2 used as the 
carrier gas. Growth was done on zincdoped GaAs single-crystal substrates. 
Dwing their work on improving device performance, this group developed an in-situ growth-rate 
mmswement technique using diffuse reflectance normal to the growth surface. The technique allows for 
mmswement of real-time surface roughness, in addition to growth rate. The period of interference fringes 
obtain& from the diffuse reflectance was compared with cross sections measured by a scanning-electron 
microscop and shown to be inversely proportional to the growth rate. 
HOT-WIRE CHEMICAL VAPOR DEPOSITION 
R. %. Crmdall and coworkers developed a different chemical vapor deposition technique for amorphous 
silicon (4). Called hot-wire-assisted chemical vapor deposition (HWCVD), it appears to be capable of 
prduchg hydrogenated amorphous silicon with properties superior to those fabricated by the more 
convenhonal plasma-enhanced chemical vapor deposition (PECVD). The HWCVD films are deposited 
using a hot filament temperature of approximately 1 ,900°C, to dissociate the silane gas, and a chamber 
presswe Pow enough to minimize gas collisions before the film precursors hit the substrate. The amorphous 
silicon films are fabricated using a S i m  gas flow rate of 20 sccm and either glass or silicon substrates held 
at varying temperatures between 40°C and 450°C. Deposition rates tended to be as much as four times 
faster than PECVD rates; mpid deposition rates are critical to the cost effectiveness of production 
techologies. Figure 1 is a schematic of this process. 
ELECTRODEPOSITION 
Instead of chemical vapors, R. Noufi, R. Bhattacharya, and coworkers developed an electrodeposition 
techique using chemical solutions for depositing superconducting films (5). Electrodeposition is an 
e1i;c~whemical process where materials (e-g., metals or oxides) in the proper stoichiometry are deposited 
on condwting substrates from chemical solutions containing the ions of interest (e.g., cu2+, y3+, ~ a ~ + ,  in 
the case of one of the high-temperature superconductors). As is almost always the case for 
el~trsd%gosition, one of the two phases contributing to an interface of interest will be a liquid electrolyte, 
which is merely a phase through which charge is carried by the movement of ions. The second phase at the 
bm* is an solid electrode (the substrate in this case), which is the phase through which charge is 
cmiesl lay electron movement. In general, when the potential of an electrode is moved from its equilibrium 
value toward negative potentials, the substance that will be reduced first (cu2+ + 2e- = cue) is the one with 
the least negative redox potential. For example, in a solution containing cu2+, y3+, and ~ a ~ + ,  the cu2+ is 
reduced first, followed by the reduction of y3+ as the potential of the electrode is made more negative. All 
three ions can be deposited on the surface of the electrode when the potential is negative enough. The 
relative concentrations of the constituents in the deposited films are empirically determined by the 
concentrations of the ions in the solution. 
There are other families of superconducting compounds involving Ba, Sr, Ca, Cu, O and Tl, Ba, Ca, Cu, O 
L researchers have successfully electrodeposited these compounds in their appropriate 
skcsicEome~es. A significantly high rate of fabrication, of the order of microns per minute, makes 
el%$-deposition a likely candidate for large-scale, cost-effective manufacturing of superconducting wires 
or tapes. Figure 2 is a schematic of this process for the fabrication of superconducting tapes made from 
thdl im cornpomds. 
PILOT LINE SPUTTERJNG 
Many PV researchers have concluded that sputtering does too much damage to films during fahcatlon to 
yield the optimum electronic properties needed for highefficiency photovoltaic devices. Yet T. Coutts and 
coworkers adapted this deposition technology, with known production advantages to a high-qualli(ji solar- 
cell design destined for space applications (7). Using Ar sputtering gas, InP solar cells were fahcated 
using two sputter guns and depositing successive layers of indium-tin-oxide (ITQ). The first thin layer 
provided a shallow homojunction in the InP substrate, while the second provides part of an antlreflection 
coating needed for high performance of the completed solar cell. Additional layers, patterned electrical 
contacts, and a post-deposition heat treatment provide controlled changes in the electrical and optncali 
properties of the solar cell and complete the processing of the final cell. Thuty-two solar cells, each 4 cm2 
in area, were completed in the pilot production. The histogram of efficiencies ranged from about 15 % to 
over 16%. The poject demonstrated that the sputtering process can be used for these relatively-lage-area 
devices and that the highest efficiency, 16.2%, is comparable to the highest reported from another 
production method. Further, the process can be configured for in-line production rather than batch 
production, characteristic of many CVD production processes. 
This paper has briefly described four film fabrication technologies developed by NREL researchers for 
either multilayer PV devices or high temperature superconducting coatings. Each of the reseasclh g011ps 
mentioned above has achieved distinction within its respective research community through the 
development of these particular film-fabrication processes for high-efficiency PV devices or 
superconducting coatings. For U.S. PV companies, superconductivity companies, or other companies 
interested in high-technology films and coatings, these and other NREL research groups are resources 
available through formal cooperative R&D agreements, such as CRADAs. 
REFERENCES 
( 1 ) SERI.. Your Laboratory for Materials Research, Processing, and Characterization, SERIISP -2 B -3600, 
( 1990) 
(2) J. M. Olson, S. R. Kurtz, A. E. Kibbler, and P. Faine, "Recent Advances in High Efficiency 
GaInP2IGaAs Tandem Solar Cells," Proceedings of the 2lst IEEE Photovoltaics Specialists Con~ferencs, 
( 1990) 
(3) A. E. Kibbler, S. R. Kurtz, and J. M. Olson, "Carbon Doping and Etching of MOCVD-grown GaAs, 
TnP, and Related Ternaries using CC4, Jourrzal o f  Crystal Growth, 109, p. 258, (199 1) 
(4) A. H. Mahan, J. Carapella, B. P. Nelson, I. B a l h g ,  and R. S. Crandall, "Deposition of Device Quality, 
Low N Content Amorphous Silicon," J. AppL Phy .  @ (9), P. 6728, (1991) 
(5)  R. N. Bhattacharya, R. Noufi, L. L. Roybal, R. K. Ahrenkiel, P. Parill, A. Mason, and D. Albin, Science 
and Technoloav o f  Thin Film Superconductors 2, Editors R. D. McConnell and R. Noufi, Plenunv Press, 
New York, P. 243, (1990) 
(6) R. N. Bhattacharya, P. A. Parilla, R. Noufi, P. Arendt, and N. Elliott, "YBaCuO and TlBaCaCuO 
Superconductor Thin Films via an Electrodepostion Process," Journal o f  the Electrochemical Society, m, 
No. I,  p. 67, (1992) 
(7) T. A. Gessert, X. Li, T. J. Coutts, and N. Tzafaras, "Pilot Production of 4 cm2 ITOAnP Photovolaa~c 
Cells," Proceedings of the 21at IEEE Photovoltaic Specialists Conference, (1990) 
@ HYDROGEN @ s l ~ l ~ o ~  ,I 
J 
Deposition Rate 21 0 &second 
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