Eric Jones, Sandia National Laboratories
Juris Kalejs, ASE Americas Inc
Rommel Noufi, National Renewable Energy Laboratory
Bhushan Sopori, National Renewable Energy Laboratory
- U.S. Dept. of Energy: EE/RE and OER/BES
Proceedings published as Volume 485
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
SESSION G1: III-V BASED THIN FILMS
Chair: L. Ralph Dawson
Tuesday Morning, December 2, 1997
8:30 AM *G1.1
HETERONUCLEATION OF III-V's ON Ge. J.M. Olson, W. McMahon, National Renewable Energy Laboratory, Golden, CO.
GaAs single junction and GaInP/GaAs tandem solar cells are currently being produced by MOCVD on a large scale for communication satellite power applications. Virtually all of these devices are grown on Ge substrates. While much is known about the Ge(100) surface in a UHV/MBE environment, little has been published about this surface in an MOCVD environment. Recently we have coupled an MOCVD apparatus to a conventional solid source MBE system and an analytical chamber via a UHV transfer chamber. The analytical chamber is equipped with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The MOCVD apparatus is also equipped with reflectance difference spectroscopy (RDS) and laser light scattering. In this paper we first examine the state of the initial Ge(100) surface in various AsH3/PH3/H2 atmospheres using LEED, AES and STM to interpret the in situ RD spectra. Annealed at temperatures below 57010C in H2, the Ge(100) surface slowly develops a preference for single-domain 1x2 terraces separated by a/2 steps where a is the unit cell dimension of Ge; with AsH3 this reaction is much faster. Above 570C (in H2), the surface becomes optically and structurally isotropic, i.e. an atomically rough surface. Arsine suppresses this ``surface melting'' and enhances the rate of the reverse reaction (disordered surface to ordered surface) by almost an order of magnitude. We also present studies of the initial stages of the nucleation on Ge(100) surfaces and examine the similarities and differences between PH3 and AsH3. We will show that both hydrides are effective at removing carbon from the surface of Ge and that extended exposure of the Ge surface to indium precursors has a detrimental effect on the morphology and defect structure of the heteroepilayer.
9:00 AM G1.2
MULTIJUNCTION InGaAs THERMOPHOTOVOLTAIC DEVICES. Navid S. Fatemi, Essential Research, Inc., Cleveland, OH; David M. Wilt, NASA Lewis Research Center, Cleveland, OH; Phillip P. Jenkins, Victor G. Weizer, Richard W. Hoffman, Jr., Essential Research, Inc., Cleveland, OH; Christopher S. Murray, David Riley, Westinghouse Electric Corporation, West Mifflin, PA; and David Scheiman, NYMA, Inc., Brook Park, OH.
We have developed multijunction monolithic interconnected module (MIM) InGaAs/InP structures for thermophotovoltaic (TPV) applications. A MIM device is comprised of several InGaAs cells connected in series on a single, semi-insulating (S.I.) InP substrate. InGaAs MIM devices lattice-matched to InP, with a bandgap (Eg) of 0.74 eV, as well as lattice-mismatched devices, with Eg=0.55 eV, were fabricated. A Au infrared (IR) back surface reflector (BSR), deposited on the back of the S.I. InP substrate, was also used to return the unused portion of the TPV emitter output spectrum back to the emitter for recycling. The advantages of MIM devices include: high voltage/low current configuration, reduced resistive losses, reduced dependence on the minority carrier lifetime, reduced view factor losses, improved reliability, and simplified thermal management and system design. Prototype devices (1x1 cm), with eight (8) series-connected InGaAs cells lattice-matched to InP, have demonstrated open-circuit voltage of 3.4 volts and a fill factor of 68% at a short-circuit current of 70 mA. Mid-infrared wavelength (>2 micron) reflectivity measurements of these devices have indicated a reflectivity of > 85%. Baseline conventional one-junction planar lattice-matched p/n InGaAs/InP cells were also fabricated. These cells demonstrated a record conversion efficiency of 11.9% under the AM0, one sun, spectrum. Recent device electrical and optical performance characteristics will be presented.
9:15 AM G1.3
DETAILED KINETICS MODELING OF INDIUM PHOSPHIDE FILMS IN MOCVD REACTORS. Maurizio Masi, Carlo Cavallotti, Guido Radaelli, Sergio Carra', Dept. Chimica Fisica Applicata, Politecnico di Milano, Milano, ITALY.
The epitaxial InP deposition on (100) surface by In(CH3)3 and PH3 in MOCVD reactors is here examined with the aim to correlate the film properties to the process parameters. In particular, a detailed deposition mechanism, based on gas phase and surface elementary reactions, is discussed. The main feature of the above mechanism is the dual-site adsorption of the precursors over the growing surface. The rate constants not determined experimentally have been estimated by means of thermochemical methods. The kinetic scheme is embedded in a 1D model of a horizontal cold wall reactor based on the boundary layer theory. Several experimental conditions of different authors are examined to validate the model prediction over the maximum possible of experimental trends. The overall model agree with the experimental data both in the case of kinetically and of mass transport controlled regimes. Surface roughness aspects have been also correlated to phosporous coverage of the surface during growth.
9:30 AM G1.4
A STUDY OF SUBSTRATE ORIENTATION DEPENDENCE OF THE SOLID-PHASE EPITAXIAL GROWTH OF AMORPHIZED GaAs. K.B. Belay, M.C. Ridgway, Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, AUSTRALIA. D.J. Llewellyn, Electron Microscopy Unit, Research School of Biological Sciences, Institute of Advanced Studies, The Australian National University, Canberra, AUSTRALIA.
In-situ transmission electron microscopy (TEM) has been utilized in conjunction with conventional ex-situ Rutherford backscattering spectrometry and channeling (RBS/C), in-situ time resolved reflectivity (TRR) and ex-situ TEM to study the influence of substrate orientation on the solid-phase epitaxial growth (SPEG) of amorphized GaAs. A thin amorphous layer was produced on semi-insulating (100), (110) and (111) GaAs substrates by ion implantation of 200 keV As and 190 keV Ga ions to a fluence of lel4/cm2. During implantation, substrates were maintained at liquid nitrogen temperature, in-situ annealing at 260C was performed in the electron microscope and the data obtained was quantitatively analyzed. It has been demonstrated that the non-planarity of the amorphous-crystalline (a/c)-interface was greatest for the (111) substrate orientation and least for the (110) substrate orientation. The roughness was measured in terms of the length of the a/c-interface in given window as a function of depth on a frame captured from the recorded video of the in-situ TEM experiment. The length or equivalently the roughness of the interface was determined by the size of the angle subtended by the microtwins with respect to the interface. The angle was both calculated and measured and was the largest in the case of (111) plane, thus confirming the a/c-interface for (111) orientation was the roughest. The twinned fraction as a function of orientation, was calculated in terms of the disorder measured from the RBS/C and it was greatest for the (111) plane. In-situ TRR and in-situ TEM data were also used to measure and compare the regrowth rate of SPE of the single crystal and twinned regimes for the different orientations considered.
9:45 AM G1.5
THE ROLE OF THE Zn INTERSTITIAL DEFECT IN HIGH EFFICIENCY P/N InP SOLAR CELLS. Richard W. Hoffman Jr., Navid S. Fatemi, Victor G. Weizer, and Phillip P. Jenkins Essential Research, Inc., Cleveland, OH; Steven A. Ringel, Ohio State University, Columbus, OH; David A. Scheiman, NYMA Setar, Cleveland, OH; David M. Wilt and David J. Brinker, National Aeronautics and Space Administration, Lewis Research Center, Cleveland, OH.
The bandgap of InP is near ideal for the high theoretical conversion efficiency of the air mass zero (AM0) light spectrum experienced in space. This combined with the well known radiation resistance makes InP an ideal candidate for space solar cell use. The record AM0 conversion efficiency value for InP is 19% and was achieved using an n/p configuration epitaxially grown on a InP single crystal substrate. The InP substrates are expensive and fragile which has contributed to the lack of acceptance and use of InP in the present space solar cell market. Hetero-epitaxial growth of InP on Si or Ge is an attractive method to reduce the cost and increase the strength of the substrates, however, this presents several challenges to the growth high efficiency cells. The lattice miss-matched growth (about 4% on Ge and 8% on Si) causes line defects which reduce the cell performance. In addition, out diffusion of the group IV substrate atoms during growth at elevated temperature causes a blocking diode to be formed when using the n/p cell configuration. Therefore, one must complicate the n/p cell structure with a tunnel junction between the active cell region and the substrate. An alternative is to use the p/n configuration. The conversion efficiency of p/n InP cells has been low compared to the n/p configuration, mostly do to poor performance of the Zn doped emitter. We have recently achieved a 17.6% AM0 conversion efficiency for a p/n cell on an InP substrate and have achieved this through a detailed understanding of the role of Zn interstitial defects and the hydrogen interactions with these defects. The effect of the Zn defect on device performance will be described.
10:30 AM G1.6
GaP TEXTURED p-n JUNCTIONS PREPARED BY LIQUID PHASE EPITAXY. Xiangyang Mei, T. Berdinskikh, H.E. Ruda, M. Buchanan*, Department of Metallurgy and Materials Sciences, University of Toronto, *National Research Council of Canada, IMS, Ottawa, CANADA.
For both photo-voltaic and radio-voltaic devices, short minority carrier diffusion lengths (compared with the absorption length and/or radiation penetration depth) are often the major factor that limit device efficiency. This is the first report of a new textured p-n junction structure which can dramatically increase the p-n junction area, and hence significantly increase carrier collection efficiency. This textured p-n junction structure was realized on n-type GaP (111)B substrates using photo-lithography, followed by wet chemical etching and liquid phase epitaxy (LPE) growth. After patterning and chemical etching, periodically arranged pyramids with height of 8m, base dimension of 10x10m and period of 18m were formed on a (111)B GaP surface. GaP epitaxial layers with different thickness' were then grown on these pyramid-covered surfaces using LPE. Scanning Electron Microscopy (SEM) images show that during the initial stages of growth, deposition occurred mainly on the side walls of the pyramids and on the bottom surfaces between the pyramids, resulting in the wells between the pyramids becoming gradually filled. Once the wells were filled, the layer surface was flat. Cross-sectional SEM images clearly show a sharp textured p-n junction interface, and electron beam induced current images show coincident with the textured interface, a region of increased charge collection efficiency. Comparing LPE layers grown under the same growth conditions on planar surfaces and pyramidal surfaces, SEM analysis shows that the total thickness of the layer over the pyramidal surface is almost the same as that on the flat surface. This indicates that growth on the tops of the pyramids had hardly occurred before the wells between the pyramids were completely filled.
10:45 AM G1.7
EFFECT OF LOCAL DEFECTS ON THE CHARACTERISTICS OF InP/InGaAs SOLAR CELLS. L.B. Karlina, B. Ya. Ber, Yu.A. Kudriavtsev, V.A. Solov'ev, C. Vargas-Aburto*, R.M. Uribe, Ioffe Physical-Technical Institute of RAS, St.-Petersburg, RUSSIA *Kent State University, School of Technology, Kent, OH.
The paper reports effect of local defects on interface substrate InP - active layer InGaAs on the characteristics of InP/InGaAs solar cells. High recombination (emitter/window) typically shows itself in a large decrease in the quantum efficiency and reduction in short circuit current density (Isc). High recombination at the lower interface usually increase in the device dark current and corresponding decrease in the open-circuit voltage (Voice). For InP/InGaAs solar cells, it is important to know the interface state density distribution at heterointerfaces, especially for monolithic tandem cells. The first type structure (n-p) consists of the n-InP transparent substrate-window and two epitaxial layers: n-InGaAs, p (Mn)InGaAs. The second type structures(p-n) consist of n InP substrate and two InGaAs epitaxial layers, undoped base and emitter, doped by manganese or zinc. The designated illumination area in the cells was 0.13 cm2. In this paper we report two ways of decrease induced defects. At first, the growth thin undoped InP layer between window - emitter or base-substrate and second way is doping - InGaAs layers by transition metal-rhenium. The effects of cell structure on the radiation response of this cell were considered in detail. The degradation of the PV parameters, the defect introduction rates, and the carrier removal rate were determined. The diffusion length damage coefficient has also been calculated. The results are compared to those of irradiated different types cells to determine how the interface and impurities affect the cell radiation response. Such information is needed to develop radiation response of high efficiency monolithic InP/InGaAs tandem solar cells.
11:00 AM G1.8
VERY HIGH HOLE CONCENTRATIONS IN C-DOPED InGaAs USING NEW SOURCES WITH AP-OMVPE. A. Tandon and R.M. Cohen, Materials Science and Engineering Dept., Univ. of Utah, Salt Lake City, UT.
New sources have been used to grow the first carbon doped, highly p-type InxGa1-xAs (0.3 < x < 0.7) epilayers on InP substrates by atmospheric pressure organometallic vapor phase epitaxy (APOMVPE). Excellent morphology was obtained simultaneously with high hole concentrations at growth temperatures near 450C. High hole concentrations of 1.6x1019-8.7x1019cm-3 (the highest reported to date for APOMVPE), and corresponding room temperature hole mobilities of 65-25 cm2/s, respectively, have been obtained from Hall measurements. X-ray diffraction is consistent with excellent crystal quality. Annealing with N2 after growth causes the carrier concentration to increase only about 15%. This is in sharp contrast to the order of magnitude changes reported by other groups who have concluded that hydrogen passivation is a major problem for the growth of C-doped InGaAs. The difference may be related to the chemical sources used. The effects of growth temperature, V/III ratio and dopant partial pressure on the carrier concentration, mobility, and growth efficiency are reported.
11:15 AM G1.9
MICROSCOPIC STUDY OF HETEROEPITAXY OF GaAs ON Ge(001). S. Gan, L. Li, R. F. Hicks, Chemical Engineering Dept., UCLA.
To understand and control heteroepitaxy of GaAs on Ge, it is important to study surfaces of germanium and of grown GaAs films. Combining STM with XPS and LEED, we investigated two most important epitaxial process in fabricating GaAs/Ge solar cells, As-passivation and GaAs film growth. Our study shows that the passivation step has a profound effect on surfaces of both Ge substrates and GaAs films. To obtain an atomically clean and smooth substrate surface, we developed a new procedure: annealing the substrates in flowing tertiarybutylarsine(TBAs) and H2 at 948K for 30min in an MOVPE reactor after cleaning them in HF and H2O2 solutions. Our experimental data indicate that TBAs annealing smoothens out the surface and reduces carbon impurities. Compared to a mountain-and-valley structure seen after the chemical etching, STM of flat Ge(001) reveals a smooth surface with both A and B steps after the TBAs annealing. On vicinal Ge(001) (9 off axis), atomic resolution STM images show a single domain structure passivated by As with arsenic dimer rows running parallel to step edges. GaAs films of 0.5 um thick were grown on this As-passivated vicinal surface. Depending on growth conditions, single crystal films of different surface morphology have been obtained. But under the ideal growth condition, films grown on well-passivated substrates of single domains display a 2D growth while a 3D growth is always observed for films grown on substrates of rough morphology.
11:30 AM G1.10
DEVICE-QUALITY GALLIUM ARSENIDE GROWN ON OFFCUT GE SUBSTRATES BY MOLECULAR BEAM EPITAXY FOR SOLAR CELLS. R.M. Sieg, S.A. Ringel, The Ohio State University, Department of Electrical Engineering, Columbus, OH; S.M. Ting, E.A. Fitzgerald, Massachusetts Institute of Technology, Cambridge, MA.
III-V multi-junction solar cell structures grown on Ge substrates are of great current interest as the devices of choice for new space satellite power applications. However, performance of such large area devices depends critically on elimination of crystalline defects such as anti-phase domains (APDís), and on control of Ge outdiffusion into the growing III-V films where the Ge acts as an n-type dopant. Toward that end, we have undertaken an extensive study of the nucleation of GaAs on Ge substrates, in an effort to eliminate APDís and minimize Ge outdiffusion. We nucleate the GaAs using molecular beam epitaxy (MBE), which allows us to characterize the surface in-situ via reflection high energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). Additional ex-situ characterization via transmission electron microscopy (TEM), capacitance-voltage (C-V), and deep level transient spectroscopy (DLTS) permits full characterization of the crystalline and electrical film properties. We have identified key steps for obtaining fully APD-free GaAs growth, which include deposition of an epitaxial Ge buffer, annealing of that buffer at 640 degrees centigrade, and initial GaAs nucleation within an optimal substrate temperature window. Analysis of the RHEED demonstrates that significant atomic exchange occurs, which drives the interface toward a low energy configuration which is independent of the initial exposure species (As or Ga). Negligible Ge outdiffusion is also obtainable. Suppression of Ge outdiffusion is accomplished via low temperature nucleation coupled with GaAs nucleation via migration enhanced epitaxy (MEE), which prevents Ge surface segregation. Additionally, DLTS-measured trap densities are comparable to homoepitaxial MBE-grown GaAs. Having achieved reproducible device-quality GaAs growth on Ge substrates, we are now extending this work toward growth of GaAs/GexSi1-x/Si structures which utilize this optimized GaAs/Ge interface. Reports of this work, as well as ongoing development of optimized MBE GaAs/Ge solar cells, will be reported at the meeting.
11:45 AM G1.11
PICOSECOND TRANSIENT PHOTOLUMINESCENCE OF GALLIUM ARSENIDE SOLAR CELLS. Steven C. Moss, Stephen D. LaLumondiere, and David M. Scott, The Aerospace Corporation, Electronics Technology Center, Los Angeles, CA.
We have used picosecond transient photoluminescence techniques to characterize advanced GaAs solar cells. The active regions of each of the solar cells consists of a p-type emitter region fabricated on an n-type base region. The active portion of the device was grown on top of an n-type buffer layer on an n-type GaAs substrate. We observe three regimes within the photoluminescence transients: (a) a fast response characteristic of minority carrier transport within the p-type emitter region, (b) a slower response characteristic of minority carrier transport within the n-type base region, and (c ) a much slower response that we attribute to emission of carriers from traps. Results were obtained as a function of base doping level and emitter layer thickness. We compare these results with picosecond transient photoluminescence measurements on double heterostructure materials at similar doping levels. We discuss the effects of optical intensity on picosecond transient photoluminescence measurements. We also discuss the limitations of the technique for extracting useful information about device response.
12:00 PM G1.12
SYNTHESIS OF LARGE-GRAINED POLY-GE TEMPLATES BY SELECTIVE NUCLEATION AND SOLID PHASE EPITAXY FOR GaAS SOLAR CELLS ON SODA-LIME GLASS. Harry A. Atwater, Jimmy C.M. Yang, and Claudine M. Chen, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA.
Despite high single-crystal cell efficiencies and a mature heterojunction technology in III-V compounds, these advantages have not yielded a efficient polycrystalline GaAs thin film solar cell, owing to high sensitivity of cell performance to crystallographic defects. Thus efficient GaAs compound polycrystalline thin film cells will require a dramatic reduction in the defect density and increase in grain size in GaAs films on low-cost substrates. Selective nucleation and solid phase epitaxy (SNSPE) is a method that yields large-grained (20-40 m) polycrystalline Ge templates for GaAs heteroepitaxy. We demonstrated recently an SNSPE process for large-grained Ge on the Sn-coated side of standard, commercially-available soda-lime glass without a diffusion barrier at low temperatures (T < 425 C). The grain size, Ge crystallization kinetics and film microstructure are very similar to those previously observed for P-doped films on thermally-oxidized Si substrates, implying that SNSPE process is unaffected by impurities in the glass or the microstructure of the Ge/glass interface. Now the principal challenge to further increasing grain size appears be understanding how grain size enhancement is related to the variation of solid phase epitaxy rate with electronically active dopant (P, B, Al) concentration. This issue, as well as the structural and optical quality of heteroepitaxial GaAs films grown on SNSPE Ge templates will be discussed.
SESSION G2: Si BASED THIN FILMS -A
Chair: Juris P. Kalejs
Wednesday Morning, December 3, 1997
8:30 AM *G2.1
DEVELOPMENT OF HIGH EFFICIENCY THIN FILM POLYCRYSTALLINE SILICON SOLAR CELLS USING VEST PROCESS. T. Ishihara, S. Arimoto, H. Morikawa, Y. Nishimoto, Y. Kawama, A. Takami, S. Hamamoto, H. Naomoto, and K. Namba. Nakatsugawa Works, Mitsubishi Electric Corporation, Hyogo, JAPAN.
Thin film Si solar cell with 16% efficiency has been developed using VEST(Via-hole Etching for the Separation of Thin films) process. The process is based on SOI technology of zone melting recrystallization(ZMR) followed by CVD, separation of thin film, and screen printing. The VEST cell has many via-holes for the separation, therefore, not only base but emitter electrodes are placed together on the rear surface. Key points for achieving high efficiency are (l)quality of Si films, (2)rear surface emitter, (3)BSF layer thickness and its resistivity, (4)front surface emitter etch-back process, and (5)hydrogen implantation. In order to improve crystal quality, ZMR process conditions were investigated. It was found that the slower the scanning speed became, the higher the quality was, and when the thickness became thinner the scanning speed for obtaining same quality was possible to increase[l]. Secondly rear surface emitter area dependence on Voc and Jsc was investigated. Voc increased 4% and Jsc 13% with increasing ratio of emitter in rear surface from 22% to 73%. From quantum efficiency measurements, long wavelength response was improved with increasing rear emitter area. It was supported from PC1D simulation with some assumption for back side emitter. Thirdly thickness and resistivity of BSF layer formed by CVD on ZMR seeding layer was studied. We can't use very low resistivity BSF to prevent the formation of p +/n+ short pass at rear side junction. So we introduce relatively high resistivity(0.1 cm) but thick BSF, which was easily realized in CVD process. The BSF thickness was varied keeping total thickness constant at 80m. While Voc increased with increasing BSF thickness up to 60m, Jsc showed maximum at 50m and the efficiency also. We introduced emitter etch back process for the improvement of short wavelength response and hydrogen ion implantation for defect passivation of Si film. As a result, we have achieved 16% efficiency(Voc:0.589V, Jsc:35.6mA/cm2, F.F.:0.763) with a cell size of 95.8cm2 and the thickness of 77n. It is the highest efficiency ever reported for large area thin film Si solar cells.
9:00 AM G2.2
SILICON SOLAR CELLS WITH POROUS SILICON LAYERS. Viktor Vikulov, Andrew Verba, Yurij Kirichenko, Dept of Radiophysics, Shevchenko Univ, Kiev, UKRAINE.
Structural, electrical and optical properties of porous silicon layer permit to consider this material as promissing for photovoltaics applications. In this work we present the solar cells with porous silicon of two types. The feasibility of the improvement of silicon cells is calculated and verified at fabrication of porous silicon antireflection layer on thick n+- emitter of p-n+ junction. It is expected of 25-29 increasing of photocurrent of silicon cell with porous silicon layer due to reduction of reflectivity from emitter side and to enhancement of the light transmission through the thick emitter. However, the absorption losses in porous silicon and 50 increase of surface recombination velocity due to this layer formation suppres effect. The methods of the reducing of recombination and absorption losses are considered. For the first time Ti (100A)-porous silicon (20A)-p-Si heterostructures have been elaborated and tested. The change of porous silicon thickness from 5 up to 60A leads to 2 time increasing of the open circuit voltage Voc. Three time improvement of short circuit current Isc can be achieved at maximum for the thickness of about 20A. A further increase in the thickness causes reducing in Isc. These dependencies are the same as for MIS solar cell with tunnel oxide layer. The additional UV illumination during 200 min (mercury lamp of low pressure) is shown to lead to enhancement of surface recombination velocity, and to the reduction of Voc and nonmonotone change in Isc. The behavior of Voc and Isc can be explained by modification of porous silicon. IR-spectra show the increase of Si-O absorption peaks (in 1030-1170 cm-1 range) and the decrease of Si-H absorption peaks (in 2080-2140 cm-1 range) after UV-illumination of cell. The main designs of silicon solar cells with porous silicon are proposed and discussed.
9:15 AM G2.3
ION BEAM DEPOSITION OF EPITAXIAL SILICON FILMS. H.R. Khan, Material Physics, FEM, Schwaebisch Gmeund, GERMANY; H. Frey, Loet-und Schweibgeraete GmbH, Aichwald, GERMANY; F. Banhart, Max-Planck Institut fur Metallforschung, Institut fur Physik, Stuttgart, GERMANY.
Thin epitaxial silicon films of <111> orientation and of 300 nm thickness are deposited on Si <111> substrates using SiH4 gas plasma and Ion Beam technique. The substrate temperature is kept at 520C and the ions are accelerated to an energy of 40 eV during the film deposition. The deposited films are characterized by X-ray Diffraction and Pole Figure analysis techniques. The morphology of the films is also investigated by Transmission Electron Microscopy. Films are found to be epitaxially grown with <111> orientation with no impurities. But the formation of the twins and defects in the films formed during the deposition are observed. The effect of deposition parameters on the Si-film growth as well as structure and morphology will be discussed.
9:30 AM G2.4
SURFACE MORPHOLOGY OF LPE SiGe LAYERS GROWN ON (100)Si SUBSTRATES. A. M. Sembian, I. Silier, K. Davies, K. Lyutovich, M. Konuma, Max-Planck Institut fuer Feskoerperforschung, Stuttgart, GERMANY; F. Banhart, Max-Planck Institut fuer Metallforschung, Stuttgart, GERMANY.
The SiGe alloy is receiving an increasing attention in the photovoltaic technology due to its characteristics such as increasing the solar cell response in the near infrared region of the solar spectrum and rendering the possibility of realizing an inverse Auger generation process for the efficiency enhancement. We have grown SiGe layers on Si(100) substrates with various concentrations of Ge by the liquid phase epitaxy (LPE) method and have studied the surface morphology of the grown layers. We observed that the growth kinetics and growth mechanisms of layer play important roles for development of the surface morphology when the SiGe layers exceed several m. Growth was performed in hydrogen atmosphere using a tipping boat LPE system. The growth temperatures were around 950C and Indium was used as a solvent. Cooling rates were kept constant during the growth and varied between 10 and 600K/h.
Continuous SiGe layers were obtained up to about 14at. of Ge concentration in the layers. Typical crosshatch patterns were observed at the surface of these layers by an optical microscope and a scanning electron microscope. The crosshatch pattern is characterized by the wavelength and amplitude of surface undulation, which we determined by a stylus surface profilometer. Atomic force microscopy revealed microscopic morphology of layer surfaces, which were characterized by concentric steps. We found that the wavelength of the undulated layers increases either upon decreasing the Ge concentration in the layer or decreasing the cooling rate during the growth process. The pyramidal growth took place when the growth was performed with Ge concentration of more than 15at. in SiGe layers.
The surface morphology of SiGe layers grown on Si(100) surface by LPE will be discussed considering the growth kinetics and mechanisms.
9:45 AM G2.5
SOLAR CELLS BASED ON THE HETEROJUNCTION a-C/p-Si. Alexander Baranov, Research Inst of Vacuum Technique, Moscow, RUSSIA; Sergei Tereshin, Inst of Radio Engineering and Electronics of the Russian Academy of Science, Moscow, RUSSIA; Yury Malov, David Zaretsky, RRC Kurchatov Inst, Moscow, RUSSIA.
Now the most dissemination found solar cells based on the crystalline silicon. However, attained in laboratories the parameters of Si- solar cells close to limit. Therefore, the task of the appliance of novel materials and the schemes of the conversion increasing efficiency of silicon solar cells excites great interest.Authors propose the novel scheme of the conversion of solar radiation to electrical energy. The scheme allows to increase short-circuit current owing to the impact ionization and broadening of the optical absorption band of light in short-wave range. This is lets to increase efficiency along comparison with solar cells based on of the silicon p-n junctions and heterojunctions. The essence of the suggested scheme of conversion consists in following. At the cell formation between by two layers p - and n - type arrange intermediate semiconductor layer in which there is strong electrical field. This layer is provided heating charge carrier and their transport without dissipation between layers n - and p - type. The photogenerated carriers obtain energy adequate for impact ionization that leads to the additional carrier generation. The parameters of the layers of heterostructure at which impact ionization can take place are discussed. Conversion unit consisted of wafer p-Si coated by the thin layer of amorphous carbon (a-C). As a top electrode was used the semiconductor of n - type from group AIIBVI. Parameters layer AIIBVI were fetched in such a way that it was antireflection coating to Si. The active area of conversion unit S=0.64 sm2. It is shown that efficiency of conversion unit 10-12 % at illumination 0.1 W/sm2 . The prospects of rising efficiency are discussed.
10:30 AM G2.6
OPTICAL CHARACTERIZATION OF ULTRATHIN SILICON SUBSTRATES. Tatiana Globus, Stephen H. Jones, Univ of Virginia, EE Department, Charlottesville, VA; and Thomas Digges, Jr., Virginia Semiconductor Inc., Fredericksburg, VA.
Ultrathin single crystal silicon substrates are needed for the micromachining industry, as well as photonic, X-ray, and optical device applications. Ultrathin silicon substrates are made to thicknesses as low as 2-4 microns. They are double-side polished, and are extremely flexible due to the dominance of the elastic nature of single crystal silicon. For many applications, the superior precision in controlling wafer planarity, surface texture, thickness uniformity is sufficient and enough. However, in other microelectronic and photonic applications there are new requirements outside of the more traditional silicon fields. Optical properties of ultrathin silicon membranes have been studied in this work to provide necessary information. The knowledge of absorption coefficient and refractive index spectra are extremely important for all devices. The results of the study presented in this paper address several major aspects. 1. Can recently developed optical interference characterization technique be accurately used for monitoring optical properties of ultrathin silicon membranes and substrates including the subgap energy range? 2. How do the optical properties of ultrathin silicon membranes and substrates correlate with optical properties of conventional crystalline silicon material? 3. What is the dependence of optical characteristics on the conductivity type and on the level of doping? 4. Is there noticeable influence of the surface treatment on optical characteristics? In this paper we present results of measurements and analysis of absorption coefficient and refractive index spectra of p- and n-type silicon membranes doped with phosphorous or boron at different levels. These results cover the wide energy range from 0.5 eV up to 2.5 eV, thus including effects of subgap defect and impurity absorption.
10:45 AM G2.7
11:00 AM G2.8
PLASMA DEPOSITION AND INTERFACE CONTROL IN LOW TEMPERATURE PROCESSING OF THIN FILM SOLAR CELLS. B. Jagannathan and W.A. Anderson, Dept. of Electrical and Computer Eng., State Univ of New York at Buffalo, Amherst, NY.
Plasma deposition of thin silicon films with a variable microstructure and controlled interface formation techniques are being developed for thin film silicon/polycrystalline silicon solar cells and also for improving the pin based amorphous cells. Low hydrogen content amorphous (a-Si:H) or microcrystalline silicon (c-Si) films are obtained by controlling the H2/Ar dilution of 2% SiH4/He in a microwave ECR discharge. The emphasis is on depositing Si films with improved stability and electro optic properties at low deposition temperatures (<400C). The films have been characterized for structural and electro-optic properties. The films prepared in this multiple dilution technique yield a-Si:H with electronic properties comparable to the rf PECVD technique and c-Si with better properties. Junction creation for solar cells was investigated using single or multilayer depositions of the film silicon onto crystalline silicon (c-Si). Interface modifications were effected by varying the microstructure of the layer in contact with the substrate or by exposing the substrates to H2 or H2/Ar plasma pretreatment. These result in various bandbending and interface formation conditions. Cells with 7% conversion efficiency (No A/R) were obtained for an a-Si/c-Si heterojunction configuration. Improved carrier transport and PV properties were achieved by inserting a thin c-Si layer in the above structure. The PV properties of such devices are dependent on the c-Si film growth conditions. Optimized cells are 9% efficient due to improvements in Jsc and FF. Improved carrier transport is also observed upon H2 plasma exposure of the substrate prior to film growth. The heterojunction characteristics are controlled by the defect states in the film layer and those at the interface created by the plasma bombardment. Capacitance spectroscopy on single layer cells indicates that the interfacial states are electron trapping sites and their nature depends on the size and energy of the impinging plasma species.
11:15 AM G2.9
DEPOSITION OF THICK SILICON LAYERS ON GLASS SUBSTRATE FOR PHOTOVOLTAIC APPLICATIONS. M. Sarret, D. Briand, K.Kis-Sion, T. Mohammed-Brahim, O. Bonnaud, GMV, URA-CNRS, Universite Rennes I, Campus de Beaulieu, Rennes Cedex, FRANCE.
A one-substrate process chamber, dedicated to deposition of silicon from silane pyrolysis and working at subatmospherical pressure has been fabricated in our laboratory. For solar cells applications, silicon layers are deposited amorphous on Corning glass substrates 1737 and subsequently annealed to perform the crystallisation. Silane is diluted in hydrogen and their flowrate are respectively 0.5 slm and 8 slm. The total pressure is 160 Torr. Amorphous layers with a thickness 10 m can be deposited within an hour. This order of magnitude of the growthrate is satisfying to obtain thick layers suitable to polysilicon photovoltaic devices. Crystalline and electrical properties of these films are compared with those of silicon layers deposited in a conventional LPCVD reactor, where the deposition rate is typically 0.3 mh.
11:30 AM G2.10
SiGe THIN-FILM STRUCTURES FOR SOLAR CELLS. Georges Brémond, Anis Daami, André Laugier, INSA de Lyon, Lab de Physique de la Matière (UMR CNRS 5511), Villeurbanne, FRANCE; Winfried Seifert, Martin Kittler, Inst for Semiconductor Physics, Frankfurt, GERMANY; Jef Poortmans, M. Caymax, IMEC vzw, Leuven, BELGIUM; Mitsuharu Konuma, Astrid Gutjahr, Inge Silier, Max Planck Inst für Festkörperforschung, Stuttgart, GERMANY.
An increasing interest is noted for recent years in the application of thin crystalline films for Si solar cell where stable efficiencies are expected and found. Such a field activity opens the route to the use of Si-based material with lower (SiGe) or higher (SiC, porous Si) bandgap than Si. These emergent new thin film structures allow an enhancement in the efficiency of solar cells by applying advanced physical properties such as higher absorption, internal field collection, surface passivation, light confinement...For instance the development and applicability of thin relaxed SiGe films as the base of a solar cell require a growth of this material with low recombination defects. This study concerns the characterization of relaxed SiGe layers grown for solar cell application.......We have characterized them using TEM, PL spectroscopy, DLTS and EBIC measurements. Graded and non-graded buffer layers in Ge concentration have been comparatively studied in order to show their influence on the reductio of the misfit dislocations in the active base layer. Also, remote plasma hydrogenation has been employed in order to decrease the non-radiative recombination pathways induced by dislocations. Our results show a clear enhancement in the morphological, the optical and the electrical quality of SiGe grown on graded layers the role of which is to confine most dislocations in the buffer layer. We show also that hydrogenation has very strong positive effect on CVD layers while LPE layers are less susceptible to hydrogenation. We interpret this behaviour in terms of differences in the dislocation recombination activity depending on the growth conditions. As a conclusion, this study points out the importance in controlling relaxed SiGe layers with good minority recombination quality as a key issue for the optimization of new SiGe/Si based solar cells.
11:45 AM G2.11
HIGH EFFICIENT ITO/Si INFRARED SENSOR MADE BY Nd:YAG PULSED LASER DEPOSITION. T.J. Zehnder, A.M. Grishin, and K.V. Rao, Department of Condensed Matters Physics, Royal Institute of Technology, Stockholm, SWEDEN.
Nanocrystalline 400 am thick indium tin oxide films (90 mol % In2O3 + 10 mol % SnO2) have been grown on boron implanted p-Si (orientation (111 ), dose 5-1015 cm^-2, energy 90 keV) by Nd:YAG pulsed laser ablation at an energy of 0.75 J/cm^-2. The substrate temperature was kept at 400^C, and the oxygen pressure during deposition was around 13.2 Pa. Then-lTO/SiO_2/p-Si junction is found to give a large photovoltaic response of V_oc= 0.34 V, J_sc= 0.66 mA/cm^2on 4.5 mW Ga-P LED= 0.93n-ITO/p-Si heterojunction can be increased almost four times by delivering a transport current of the order of one mA/cm^2through the IR sensor. A sharp maximum of the photoresponse is achieved at the saturation of the reverse current which drops back at sufficiently higher fields as a result of avalanche carrier multiplication. Reverse bias, separating photoinduced electron-hole pairs, is also found to significantly reduce the recombination processes. To demonstrate this, the temporal dispersion of the photoresponse on the cropped He-Ne radiation (8 mW at= 632.8 nm) has been measured as a function of the bias current. The recombination time is found to increase by a factor of two, reaching a value of 90s, at the same saturated reverse current which results in sharp maximum of the photoresponse. The above described features are a specific feature of ambipolar diffusion of carriers. In brief,nandp-type carriers, induced by the light generation, create the photovoltage drop across the heterojunction. A small recombination current competes with this effect and can be compensated by the transport bias current through the junction. Such double charged depletion layer becomes extremely sensitive to the low power radiation if this current equilibrium could be achieved.
SESSION G3: Si BASED THIN FILMS - B
Chair: Takashi Ishihara
Wednesday Afternoon, December 3, 1997
1:30 PM *G3.1
NANOCRYSTALLINE SILICON/AMORPHOUS SILICON DIOXIDE SUPERLATTICES. Philippe M. Fauchet, Leonid Tsybeskov, and Margit Zacharias, Department of Electrical Engineering, Computer Studies Building, University of Rochester, Rochester, NY.
Thin layers made of densely packed silicon nanocrystals sandwiched between amorphous silicon dioxide layers have been manufactured and characterized, We first grow an amorphous silicon/amorphous silicon dioxide superlattice by CVD or sputtering. The a-Si layers are recrystallized in a two-step procedure (nucleation + growth) to form a plane of nearly identical nanocrystals whose diameter is given by the initial a-Si layer thickness. The recrystallization is monitored using a variety of techniques, including TEM, X-Ray, Raman, and luminescence spectroscopies. The recrystallization temperature increases dramatically compared to that of a single a-Si film, with decreasing a-Si layer thickness and/or increasing a-SiO2 layer thickness. We have not yet been able to fully recrystallize the thinnest a-Si films ( 3 nm), which contain nanocrystals of unusual shapes. We will discuss how to remove the a-Si tissue present between the nanocrystals, how to passivate the nanocrystals, and how to dope them. Finally, the optical properties, the electronic properties, and the optoelectronic applications of these nc-Si/SiO2 superlattices will be discussed.
2:00 PM G3.2
Si SURFACE PREPARATION WITH Si BEAM IRRADIATION ON THE GROWTH OF III-V ON Si. H. Kawanami, I. Sakata, and T. Sekigawa, Electrotechnical Laboratory, Ibaraki, JAPAN.
III-V on Si is the most promising material structures for high-efficiency solar cells because high quality, low cost and large area Si substrate can be used. However, the III-V on Si system has many problems such as the epitaxial growth on highly mismatched substrates, surface preparation just before the film growth at a moderate temperature. It is well known that epitaxial Si substrate (ESS) has superior effects on the III-V on Si . The Si surface structure is expected to be well controlled by the Si epitaxial process. Here we report the preliminary results of the effects of the Si beam irradiation for the surface preparation i.e. the effects of the epitaxial process on the growth of III-V on Si by MBE. It was observed that Si beam irradiation for 30 min. at moderate temperature (- 800C) results in a single domain Si (001) surface reconstruction. GaAs films were grown on the Si surface to understand the effect of Si surface preparation. The quality of the grown films were estimated by surface morphology, XRD and PL. Si beam assisted thermal desorption at high temperature (-800C) has a positive effect on GaAs film growth. Longer time and low temperature Si irradiation causes deterioration of the quality of the grown film. It might be due to contamination of substrate surface from the ambient gases because of slower growth rate of Si.
2:15 PM G3.3
POLYCRYSTALLINE Si FILMS FABRICATED BY LOW TEMPERATURE SELECTIVE NUCLEATION AND SOLID PHASE EPITAXY PROCESS. Claudine M. Chen and Harry A. Atwater, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA.
Future thin film poly-Si solar cells on low-cost substrates will require methods for fabrication of approximately 10-30 m thick Si films with large (30-100 m) grain sizes at low temperatures. With a selective nucleation and solid phase epitaxy (SNSPE) process, grain sizes of 10 m have been achieved to date at 600C in 100 nm Si thick films on amorphous SiO2, with potential for greater grain sizes. Selective nucleation occurs via a thin film reaction between a patterned array of 20 nm thick In islands which act as heterogeneous nucleation sites on the amorphous Si starting material. Crystal growth proceeds by lateral solid phase epitaxy from the nucleation sites, during the incubation time for random nucleation. The largest achievable grain size by SNSPE is thus approximately the product of the incubation time and the solid phase epitaxy rate. Electronic dopants such as B, P, and Al, are found to enhance the solid phase epitaxy rate at high concentrations: the growth rate of Al-doped Si (peak conc. 1.01019/cm3) was enhanced by a factor of 2 relative to intrinsic Si, P-doped Si (peak conc. 2.01020/cm3) had an enhancement factor of 27 relative to intrinsic Si, and B-doped Si (peak conc. 2.01020/cm3) had an enhancement factor of over 50 relative to intrinsic Si. The optimum doping concentration to produce large grain size by SNSPE is constrained by several competing factors, including the electronic enhancement effect, doping segregation at the amorphous-crystal interface, and unwanted heterogeneous nucleation resulting from dopant precipitation at concentrations above the solid solubility limit. SNSPE results from doped Si with lower impurity concentrations, and results of vertical solid phase crystallization to form 10 m thick poly-Si on soda-lime glass will be discussed.
2:30 PM G3.4
LOW-TEMPERATURE DEPOSITION OF POLY-CRYSTAL SILICON THIN-FILMS. Wen-chang YEH, Masakiyo MATSUMURA; Tokyo Inst of Technology, Dept of Physical Electronics, Tokyo, JAPAN.
We have proposed and demonstrated successfully an H-free sputtering method of Si for low-temperature deposition of poly-Si thin-films on glass. The film was deposited at temperatures as low as 325C, i.e., about 200C lower than the typical temperature for the CVD or plasma-CVD deposition of poly-Si films. Thus the proposed method seem open a new process technology aiming at high-efficiency solar-cells or high performance thin-film transistors.
Firstly, ultra-thin Ni films (of less than 4nm in thickness) were deposited on SiO2 substrate, and Si films were consecutively deposited on them with their deposition rate of about 0.6nm/s. An H-free sputtering was confirmed essential since surface-terminating H in the CVD and plasma CVD methods hinders migration of surface adsorbates, resulting in the elevated crystallization temperature. The Ni underlayer was also confirmed essential, although it should be very thin, due to its miracle catalytic effects for crystallization of Si.
XRD and RHEED measurements indicated that the highly crystallized Si film was deposited at temperatures as low as 380C. The film had a (222)-direction oriented columnar structure with average diameter of less than 50nm and with more than 1m in length.
Poly-Si pn diodes and thin-film transistors were fabricated on glass substrates. Their electrical properties will be also presented.
2:45 PM G3.5
INVESTIGATIONS OF OXIDATION DEPENDENCE ON THE TYPE OF POROUS SILICON NEAR THE ROOM TEMPERATURE. J. Salonen, V-P. Lehto, and E. Laine, Univ of Turku, Dept of Physics, Turku, FINLAND.
The oxidation of porous silicon (PS) has been studied using thermal activity monitoring, i.e., isothermal microcalorimetry. It was found that the microcalorimetric signal from the oxidation of the (p+)-type PS reduces exponentially at the room temperature (25C), while in the case of n-type PS the signal starts to increase slowly, reaching its highest value after 40 hours. This kind of behaviour is typical of autocatalytic reactions. Similar behaviour was observed with the samples placed in water, indicating the difference originates from the different structure of the n- and (p+)-type PS. To clarify the origin of the difference, the preparation parameters (e.g., current density and HF concentration) of the PS were varied to obtain samples with different porosity and structure. The activation energy was determined from the measurements near the room temperature (25-75C). In these measurements, we observed that the oxidation of the n-type PS at the higher temperatures approached to the oxidation behaviour observed in the case of the (p+)-type PS. The results of this research have been compared with the previous observations [1-3], and the origin of the difference between the oxidation of n- and (p+)-type PS has been discussed.
3:30 PM G3.6
LOW TEMPERATURE GROWTH OF CRYSTALLINE SILICON THIN FILMS BY ECR PLASMA CVD. Licai Wang and Haricharan Reehal, School of Electrical, Electronic and Information Engineering, South Bank University, London, UNITED KINGDOM.
Thin film crystalline silicon solar cells are attracting widespread attention because of their potential for low cost and high efficiency photovoltaic devices. Compared to competitor thin film technologies such as amorphous silicon and cadmium telluride, thin film crystalline silicon offers significant potential advantages including stable performance, no resource constraints and minimal environmental impact. A low cost technology will, however, require growth of crystalline silicon films on cheap substrates such as glass. This is a stringent requirement because although glass is the ideal substrate material for many reasons, its use restricts processing temperatures to below 600C.
We report some results on the growth of silicon films at temperatures 350C on glass, metal-coated glass and single crystal silicon substrates using the technique of ECR (electron cyclotron resonance) plasma-assisted CVD (chemical vapour deposition). On silicon substrates, the as-grown films are microcrystalline with grain sizes in the region of 100 nm as determined by X-ray diffraction measurements. Grain size increases with thermal annealing to values of 300 to 400 nm. The predominantly crystalline nature of the as-grown films is indicated by Raman spectroscopy.
The microstructure of as-deposited films on glass, aluminium-coated glass or palladium-coated glass varies with growth conditions and substrate type. Films on the metal-coated substrates are primarily amorphous in character. However thermal annealing at 600C leads to a marked change in the silicon Raman band at approximately 520 reciprocal cm for these films, particularly those deposited on palladium-coated substrates. The change is indicative of a significant increase in the crystalline fraction after annealing. Full details of film growth and properties as a function of growth conditions will be presented.
The use of these layers in solar cell structures will also be discussed and described.
3:45 PM G3.7
TAILORED SI-LAYERS ON SILICON OXIDE OBTAINED BY THERMAL CVD. G. Beaucarne, J. Poortmans, M. Caymax, J. Nijs, R. Mertens, IMEC, Leuven, BELGIUM.
Crystalline silicon solar cell technology is well-established and yields high and stable efficiencies. However, in order to reduce the cost, efforts are being made to make crystalline silicon cells in films deposited on foreign substrates. Two challenges have to be faced : firstly, as silicon is an indirect bandgap semiconductor, an efficient light trapping scheme has to be implemented. Secondly, the Si layers have to be of sufficient quality to ensure efficient collection of the light-generated charges. As grain boundaries contain numerous recombination centres, a major requirement is to make grains as big as possible while keeping a continuous layer. To achieve this, it is essential to control the nucleation process of Si on the foreign substrate in the early stage of deposition. In this paper, the nucleation of Si on silicon oxide surfaces in a thermal CVD reactor with a hydrogen - dichlorosilane - HCl ambient at high temperature is investigated. The nucleus density on the oxide surface is monitored in function of HCl-concentration and temperature. It is shown that the nucleus density decreases with increasing temperature. Experiments with HCl-addition to the gas flow indicate that the nuclei density is initially inversely proportional to the square of the HCl-concentration, but drops to very low values at a threshold HCl-concentration. In the paper, these results are explained in the light of atomistic nucleation theory. The effect of addition of diborane on nucleus density and boron incorporation to the gas flow is described. It is further shown that by controlling the nucleus density through the growth parameters, Si layers with the desired grain size can be obtained. Such layers are then used in a thin-film crystalline silicon solar cell process.
4:00 PM G3.8
GRAIN ENHANCEMENT OF POLYCRYSTALLINE SILICON FILMS AIDED BY OPTICAL EXCITATION. Wei Chen, Bhushan L. Sopori, National Renewable Energy Laboratory, Golden, CO; N.M.Ravindra, New Jersey Institute of Technology, Department of Physics, Newark, NJ; T.Y.Tan, Duke University, Department of Mechanical Engineering & Material Science, Durham, NC.
Thin-film silicon solar cells (about 15 m thick) can yield high efficiencies with a lower quality material if a suitable light trapping design can be incorporated. However, to maintain a cost advantage, such a cell must use a low-cost substrate such as glass with a concomitant need for low temperature processing. The low-temperature processing requirement makes it difficult to directly deposit films of large grain size (much larger than the film thickness) by conventional techniques. One approach to overcome such a limitation is to use solid-phase grain growth by low-temperature thermal heating. Unfortunately, this approach requires long annealing times. We show that the annealing times can be greatly reduced if the sample is illuminated during the process and vacancies are injected during the annealing process. We describe the results of a process in which a thin film of silicon deposited on an Al-coated glass substrate is illuminated with a broad-band light, typically 4-6 W/cm2, yielding large grains. This grain enhancement process can produce large grains, up to 20 m in size, in short periods of time. The typical process times range between 2 to 5 minutes, while the process temperatures are below 500C. The roles of heating, light illumination and vacancies injection will be discussed. We will also discuss the effects of dopants and the interface control on the enhancement process.
4:15 PM G3.9
A THIN-FILM SILICON SOLAR CELL: DESIGN AND PROCESSING APPROACH. Bhushan L. Sopori, Wei Chen, Jamal Madjdpour and Marta Symko, National Renewable Energy Laboratory, Golden, CO.
It is now well recognized that thin-film silicon solar cells can yield high efficiencies with somewhat modest material quality. However, a thin film cell must meet many very challenging requirements: an effective light-trapping to enhance the absorption of the incident solar spectrum, a low-cost supporting substrate, and a minimum impurity diffusion from the substrate during growth and processing of the Si film. Therefore, the design of a thin-film Si cell involves determination of an appropriate configuration compatible with such requirements and a methodology for calculating parameters such as film thickness, grain size, antireflection coating parameters, texture and back reflector parameters. To date, such design rules have not been established. We describe a new single-junction, thin Si cell structure and processing approach for its fabrication. A fine grain or amorphous Si film is deposited on an Al-coated glass substrate, and grain enhanced by an optical processing technique. The interfacial Al film functions as a multipurpose buffer layer; it participates in grain enhancement, acts as an electrode, serves as a gettering layer, and provides back reflection for effective light trapping. Grain enhancement is performed by annealing the film-coated substrate using optical excitation. The optimum cell parameters, determined by PV Optics, will be discussed. These design considerations show that such a structure is capable of yielding >18 efficiency with current technology.
4:30 PM G3.10
ATOMIC-SCALE ANALYSIS OF THE REACTIVITY OF RADICALS FROM SILANE/HYDROGEN PLASMAS WITH SILICON SURFACES. Shyam Ramalingam, Dimitrios Maroudas and Eray S. Aydil, Dept of Chemical Engineering, University of California-Santa Barbara, Santa Barbara, CA.
Hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon produced by PECVD on silicon substrates from silane/hydrogen plasmas are key materials in photovoltaic technologies. Fundamental understanding of plasma-surface interactions and surface reaction mechanisms is essential to produce high-quality films. Toward this end, a systematic atomic-scale analysis is presented of the reactivity of radicals originating from the silane/hydrogen plasma with surfaces of crystalline silicon substrates and a-Si:H films of different hydrogen concentrations. Our analysis is based on molecular-dynamics (MD), structural relaxation, and Monte Carlo simulations using an extension of Tersoff's potential to describe interatomic interactions in the Si:H system. The silicon surfaces were characterized in detail and the energy gain associated with the attachment of the radicals at different surface locations was calculated. MD simulations of radical impingement were carried out in which the mobility of the radicals, their ability to penetrate the different surfaces, as well as the bond-breaking and bond-forming processes that determine the chemical reactivity were monitored in detail. The analysis indicates that the radicals SiH, SiH2 and SiH3 differ in their reactivities with the different surfaces; this is in agreement with previous experimental research. In addition, the hydrogen concentration of the surface and the location of impingement on the surface of the radical were found to influence strongly both the reactivity and the surface reaction mechanism. Specifically, SiH was found to react with the a-Si:H films irrespective of location of impingement and radical orientation; the reaction mechanism, however, depends strongly on the hydrogen coverage of the surface. This result is in agreement with experimental data for the SiH reaction probability with a-Si:H films, which was measured to be higher than 94%.
4:45 PM G3.11
PHOTOVOLTAIC PROPERTIES OF FULLERENE (C60) THIN FILMS. E.A. Katz, D. Faiman, Ben-Gurion Univ of the Negev, National Solar Energy Center, Sede Boker, ISRAEL; S. Goren, S. Shtutina, A. Shames, Ben-Gurion Univ of the Negev, Dept. of Physics, Beersheba, ISRAEL; B. Mishori and Yoram Shapira, Tel-Aviv Univ, Dept. of Phys. Electronics.
We demonstrated recently that C60 is a perspective material for thin film photovoltaics. This paper presents further study of crystalline and electronic structure of C60 thin films, photoelectrical phenomena in the material as well as parameters of C60/Si heterojunction solar cells. By varying the deposition conditions of C60 films we obtained a continuum of structure types ranging from amorphous to fcc polycrystalline. We have revealed the presence of EPR active species with the same spectral parameters in as-grown and in light irradiated thin films. The paramagnetic centers attributed to C60+ radicals are located in the bulk of the thin film and their number in as-grown films depends on the growth conditions and structure of the films. The Surface Photovoltage (SPV) Spectroscopy results indicate that an electron structure of C60 thin films includes band tails extending into optical gap of the material and two deep level states in the gap. Annealing at moderate temperatures is shown to effect on the structure of the films as well as on the density of paramagnetic centers and deep gap states measured by EPR and SPV spectroscopy, respectively. A persistent photoconductivity and a persistent internal photopolarization effect are reported. Both phenomena are discussed on the base of a model of photogenerated carrier trapping at deep-level centers. Finally, parameters of C60/Si heterojunction solar cell are reported. Observed SPV spectrum of the device is considered as an evidence of that C60 acts as an active layer of photovoltaic cell.
SESSION G4: II-VI BASED THIN FILMS - A
Chair: S. Nishiwaki
Thursday Morning, December 4, 1997
8:30 AM G4.1
A STUDY OF THE CBD CdS/CuInGaSe2 INTERFACES. Kannan Ramanathan, Raghu Bhattacharya, Jennifer Granata, Miguel Contreras, James Keane and Rommel Noufi, National Renewable Energy Laboratory, Golden, CO.
The most efficient CuInGaSe2 thin film photovoltaic devices are fabricated by depositing a thin CdS film by chemical bath deposition (CBD). All other materials, including CdS deposited by other methods, do not yield optimal interfaces and, consequently, result in inferior devices. There are many reports which describe the CBD growth recipes and the CdS film properties. The physics of the interface formation, which is of paramount importance to devices, has not been studied in detail and it is the subject of this paper. We shall report on the effect of the chemical bath constituents on the optical, electrical properties of the CuInGaSe2 thin films and the photovoltaic properties of the devices. We show that the effect of Cd alone on the CIGS films is significant, and it is possible to argue that the Cd substitutes for the Cu vacancies resulting in an n-type surface. The paper will discuss the photoluminescence, surface analysis and photovoltaic properties.
8:45 AM G4.2
PHASE AND INTERFACE STABILITY ISSUES IN CHALCOPYRITE-BASED THIN FILM SOLAR CELLS. Jean-Francois Guillemoles, ENSCP, Laboratoire d'Electrochimie, Paris, FRANCE; Thomas Haalboom, Max Planck Institute, Stuttgart, GERMANY; David Cahen, Dpt of Materials and Interfaces, Weizmann Institute of Science, Rehovot, ISRAEL.
Chalcopyrite semiconductors are known not only from their success in photovoltaic energy conversion, but also as ionic copper conductors. This questions the stability of CuInSe2 solars cells. Contrarywise to copper sulfide solar cells, using the Ion Potential Diagrams, we show that in this case thermodynamic and kinetic self-stabilisation processes are operative. We also discuss the stability of the absorber layer relative to the various buffer layers used in the devices. Based on new results obtained by an intensive TEM study, we supplement the existing data on the room temperature phase diagram of CuInSe2 and we explain why no OVC overlayer separate phase could be evidenced, even though the surface composition corresponds to the I-III3-V5 compound.
9:00 AM G4.3
SURFACE MODIFICATION OF CU(IN,GA)SE2 THIN FILMS DURING WET OXIDATION ETCH. Jean-Francois Guillemoles, Daniel Lincot and Jacques Vedel; ENSCP, Laboratoire d¥Šlectrochimie, Paris, FRANCE
The control of the position of the Fermi level pinning at the Cu(In,Ga)Se2(CIGS)/CdS interface is a key to improved solar cells. We present new wet oxidative etches can be used to modify the interface, based on (a) water peroxide or (b) permanganate. In both cases, the pH can be adjusted so as to give a uniform etch controlled by the dissolution of a thin passivating layer. The resulting surface composition of CIGS is analysed by XPS. Electronic properties of the CIGS surface and CIGS/CdS interface are characterized by UPS and Surface Photovoltage Spectroscopy.
9:15 AM G4.4
CHARACTERIZATION OF Cu(InGa)Se2/Mo INTERFACE IN CIGS SOLAR CELL. S. Nishiwaki, N. Kohara, T. Negami, M. Nishitani, Y. Hashimoto and T. Wada, Central Research Laboratories, Matsushita Electric Ind. Co., Ltd., Kyoto, JAPAN.
We have demonstrated CIGS solar cells with efficiencies of over 17%. The previous study showed that there was MoSe2 layer at the interface of the high efficiency CIGS solar cells . In this symposium, we report on the characterization of the MoSe2 layer at the CIGS/Mo interface in high efficiency CIGS solar cells. A Mo back contacts were deposited on a sodalime glass by rf-magnetron sputtering. The CIGS absorber layers were deposited by the ``3-stage'' process using an in-situ composition monitoring system. The Mo/CIGS interfaces were characterized by x-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS) and analytical transmission electron microscopy (TEM). The MoSe2 layer was formed at the CIGS/Mo interface in the 2nd stage of the 3-stage process. The thickness of the MoSe2 layer depended on the substrate temperature and Na content of the CIGS layer. The higher substrate temperature and the higher Na content, the thicker MoSe2 layer was observed. The effects of MoSe2 layer on the performance of CIGS solar cells are discussed.
9:30 AM G4.5
IN-SITU SURFACE COMPOSITION MEASUREMENTS OF CuGaSe2 THIN FILMS. P. Fons,A. Yamada, S. Niki, and H. Oyanagi, Electrotechnical Laboratory, Tsukuba, JAPAN.
CuIn1-xGaxSe2 is promising material for polycrystalline photovoltaic device fabrication. Recently reported conversion efficiencies of photovoltaic devices fabricated from polycrystalline thin films of CuIn1-xGaxSe2 have approached 18%. As the bandgap of CuInSe2 is 1.04eV and that of CuGaSe2 is 1.7eV, in order to attain the highest single-cell conversion efficiency bandgap of 1.5 eV, it is necessary to alloy to high Ga concentrations. To date, however, attempts to grow polycrystalline films of large x, have not yielded the high efficiencies expected. One possible explanation is phase separation of regions with Ga content significantly deviating from the ideal 1:1:2 ratio. Additionally, due to the small volume fraction of such phases, it is difficult to do a careful analysis of second phase presence of the finished film with ex-situ techniques such as x-ray diffraction. To address the problem of possible phase separation during growth, we have grown a series of epitaxial CuGaSe2 films on GaAs (001) substrates by MBE from elemental sources at 450C using varying Cu/Ga flux ratios. All samples were grown under excess Se conditions. Each film was grown under a fixed Cu/Ga ratio. Growth was periodically interrupted to examine surface composition using in-situ Auger electron spectroscopy. As the 1/e escape depth for the Auger electrons used is 15 Å, about 95% of the resulting Auger signal arises from the top 50 Å. Measurements of the Cu/Ga surface composition during growth under growth conditions with an incident Cu/Ga flux ratio of less than unity, showed a decrease in Cu/Ga ratio from 0.85 for 400Åto 0.58 for 2000Å. RHEED observations also indicated the presence of additional spots not indexable using the chalcopyrite structure. Post growth ex-situ cross sectional TEM measurements confirmed the presence of additional phase after approximately growth of 2000 Åof what appeared to be a single phase film. Surface composition of films grown with a Cu/Ga ratio of greater than unity will presented as well.
9:45 AM G4.6
PIEZOELECTRIC PHOTOACOUSTIC SPECTRA OF CuGaSe2 THIN FILMS GROWN BY MOLECULAR BEAM EPITAXY. K. Yoshino, D. Maruoka, M. Kawahara, T. Shimizu, Department of Electrical and Electronic Engineering, Miyazaki Univ., Miyazaki, JAPAN; A. Fukuyama, Department of Materials Science, Miyazaki Univ., Miyazaki, JAPAN; K. Maeda and T. Ikari, Department of Electrical and Electronic Engineering, Miyazaki Univ., Miyazaki, JAPAN; P. J. Fons, S. Niki and A. Yamada, Electrotechnical Lab., MITI, Ibaraki, JAPAN.
CuGaSe2 (CGS) is a promising material as one of the high efficient solar cell active layers. We have reported that Cu-rich CGS epitaxial layers on substrate GaAs (001) were successfully grown by molecular beam epitaxy (MBE) at 490C . These samples were examined by means of photoluminescence (PL) and photoreflectance (PR) techniques. The free-exciton (FE) emission bands due to so called A-, B- and C-exciton transitions were clearly observed in the PR spectra at low temperatures. This indicates that the present CGS epitaxial layers are high quality. In this paper, piezoelectric-photoacoustic (PPA) spectra are measured at liquid nitrogen and room temperature in order to obtain an information of nonradiative radiation. A piezoelectric transducer (PZT) is known to be more sensitive than a gas-microphone and the former can be used even at low temperature. The four peaks are observed in the PPA spectra of Cu-rich CGS sample at liquid nitrogen temperature. It is considered that two of the four peaks are due to bandgap of GaAs and CGS and the other two peaks are due to B and C bands of CGS. The PPA signals of CGS/GaAs thin films grown by MBE are successfully observed for the first time.
10:30 AM G4.7
METALORGANIC CHEMICAL VAPOUR DEPOSITION STUDIES OF CUINSE2 FROM COPPER AND INDIUM DISELENOCARBAMATES FOR SOLAR CELL DEVICES. John McAleese, Paul O'Brien, and David J. Otway, Imperial College, Dept of Chemistry, South Kensington, London, UNITED KINGDOM.
CuInSe2 is starting to find use as a radiation hard photovoltaic material, ( for clean power generation from solar energy) but, as yet, has not been reproducibly deposited as thin films by MOCVD. There are plans to include the material on one of NASAís space vehicles to test its suitability/reliability in space environments. The deposition of thin films of CuSe, In2Se3, and CuInSe2, by MOCVD, from copper and indium diselenocarbamate (R2NCSe2-) compounds has been studied, as well as the chemistry of the precursor compounds. The thin films thus deposited have been investigated by X-ray diffraction, scanning electron microscopy and UV spectroscopy (direct band gap method).
10:45 AM G4.8
CHARACTERIZATION OF CIS THIN FILMS DEPOSITED BY PULSED ELECTRON BEAM ABLATION. Serghei Malkov and Andrei M. Andriesh, Inst of Applied Physics, Kishinev, MOLDOVA.
The results of the structural, phase, morphological and optical characterization of CuInSe2 (CIS) thin films deposited by pulsed electron beam ablation are presented. The pulsed electron beam ablation is concerned to be similar to pulsed UV laser ablation but the capital costs are many times lower. It is successfully emloied for depositing high temperature multinary superconductors, glasses, polymers and metal alloys. In this technology method a pulsed high current and magnetically self pinched electron beam produced in a low pressure channel spark camera was used for the CIS deposition at the following beam parameters and growth conditions: pulse duration - 100 ns, power density <500 MW/cm2, penetration depth into target - 0.5-1.5 mkm, argon pressure- 1-3 Pa, substrate temperature- 200-400 C, repetition rate- 1-10 Hz.
The X-Ray Diffraction measurements have indicated on the target structure reproducible transfer to the CIS thin film identifying peaks associated with 112, (220)/(204), (116)/(312) and (316)/(332) reflections. The CIS films prepared were found to have the strong preferential (112) orientation, the single phase chalcopyrite structure, the columnar grain microstructure and the surface morphology consisting of the smooth background of closely arranged grains and droplets disposed on the surface. The observed (112) fiber texture is assumed to correspond to the lateral thin film growth mechanism. It was established that the significant subband gap absorption was observed in the films deposited at low substrate temperatures but well-defined absorption edge near the band gap was measured for the films deposited at the substrate temperatures higher than 350 C.
11:00 AM G4.9
GROWTH OF Cu(InGa)Se2 FILMS BY DUAL-LASER ABLATION. Sarath Witanachchi, John Cuff, Alfred M. Miyawa, Robert Jones and Pritish Mukherjee, Laboratory for Advanced Materials Science and Technology (LAMSAT), Department of Physics, University of South Florida, Tampa, FL.
Ablation of a target material by the combination of excimer and CO2 laser pulses, that are spatially overlapped on the target and suitably delayed, produces high energy material plumes that expand rapidly in vacuum. Under the optimum conditions the films deposited by this novel dual-laser process are uniform over a large area and lack the defects that are indigenous to films deposited by conventional single laser ablation. Since the composition of a single composite multicomponent target can be easily reproduced at the substrate by the laser ablation process, this technique is ideally suited for the growth of complex multicomponent films. In this paper the advantages of the dual-laser ablation process in the growth of compound semiconductor photovoltaic films are presented. Cu(InGa)Se2 films have been grown on glass and silicon substrates by the single and dual-laser ablation processes to investigate the effect of the dual-laser process on the electrical and optical properties of these films. The morphological and structural changes induced by the presence of high energy species in the dual-laser ablated plume were studied by SEM and x-ray rocking curve techniques. Furthermore, the spatial variation of plume species was studied by in-situ optical emission spectroscopy and a correlation was made with the radial EDX scans of the deposited films. The electrical, optical and structural properties of these films will be compared with films grown by other techniques.
11:15 AM G4.10
NOVEL MULTILAYER PROCESS FOR CuInSe2 THIN FILMS FORMATION BY RAPID THERMAL PROCESSING. Chih-hung Chang, Billy Stanbery, Augusto Morrone*, Albert Davydov, Tim Anderson, Dept of Chemical Engineering, Department of Materials Science and Engineering*, University of Florida, Gainesville, FL.
Considerable effort has been devoted to developing low cost technologies for fabrication of CuInSe2-based solar cells. Rapid Thermal Processing (RTP) is an attractive techniques with the advantages of high throughput, low thermal budget, and better control of the process kinetics. Formation of CuInSe2 thin films by RTP had been reported by several groups using codeposition of Cu+In+Se solid mixture  or stacked elemental layers[2,3] as the precursor film structures. In this work, binary In-Se/Cu-Se precursors were fabricated for RTP synthesis of CuInSe2 thin films. This precursor structure was suggested based on our assessment of the Cu-In-Se ternary phase diagram. This structure suggests greater flexibility and lower temperature for the reaction pathway of CuInSe2 formation. Multilayer precursors films were deposited in our novel rotating-disc reactor at a low processing temperature (<400C). Ex-situ RTP was performed in a customized RTP furnace and a quartz susceptor capable of providing a reactive vapor ambient The process parameters (ramp rate, maximum annealing temperature, and soak time) were explored to study the reaction pathways and phase evolution. The microstructures were characterized by TEM, X-ray and electron diffraction. The compositions were measured by Energy Dispersive X-ray analysis (EDX) or Wavelength Dispersive X-ray Spectroscopy (WDS), while a thermoelectric probe was used to assess the conductivity type of the annealed films.
11:30 AM G4.11
PREPARATION OF HIGH-QUALITY CuInSe2 THIN FILMS BY MOLECULAR BEAM EPITAXY. Bae-Hong Tseng, Song-Bin Lin and Dung-Gin Yang, Institute of Materials Science and Engineering, National Sun Yat-Sen University, Taiwan, TAIWAN.
The growth of high-quality CuInSe2 epitaxial films is essential for the study of fundamental material properties as well as the development of high-efficiency solar cell. In this work, CuInSe2 films were grown on GaAs substrates by molecular beam epitaxy (MBE). We demonstrate that epitaxial films of CuInSe2 free of orientation and antiphase domain structures can be grown on (001)GaAs substrates followed by in-situ annealing the films in the presence of the Se-beam flux and the interfacial structure can be modified by the low-temperature MBE techniques (e.g. microtwins instead of dislocations prevail in the epitaxial film grown by photo-assisted MBE). Transmission electron microscopy (TEM) had been applied to verify the above-mentioned results. For the annealed films, TEM observations also show a reduction in the dislocation density in the epitaxial layer. The refinement of defect structures in CuInSe2 films result in the improvement in the film properties. Hall measurements show an increase in the carrier mobility and a decrease in the degree of compensation in annealed films. In addition, a comparison of the photoluminescence spectra indicates an increase in luminescence intensity and a change in the distribution of intrinsic defects in CuInSe2 after annealing. The films with superior luminescence properties can be obtained by a precise control of film composition since a small deviation from stoichiometry may lead to a significant decrease in the excitonic emission intensity.
11:45 AM G4.12
IN-SITU INVESTIGATION OF CU-IN-SE-REACTIONS BY THIN-FILM CALORIMETRY. Dietrich Wolf, Georg Müller, Crystal Growth Laboratory, Dept of Materials Science, Erlangen-University, GERMANY.
Among different techniques the selenization of metallic precursor layers has been proven to be an appropriate and reproducible method to produce polycrystalline CuInSe2 (CIS) thin films for photovoltaic applications. However, investigations of the formation of the ternary compound starting from the elements have been restricted up to now to post-annealing XRD-measurements. Consequently, the results of these phase analyses are assumed to be typical for specific processing conditions only and do not reflect the general reaction pathway. Targeting a detailed and well defined in-situ study of phase formation and reactions in the Cu-In-Se system we apply the Differential Scanning Calorimetry (DSC) to thin binary and ternary films. DSC of films with a thickness of practical interest for CIS absorbers requires low thermal resistivities between the specimen and the detection system. By thermally evaporating the elements directly into metallic sample pans which are hermetically sealed under different atmospheres we are able to maximize resolution and sensitivity of the DSC-system. The potential of this method is demonstrated by resolving the melting reaction of In films as thin as 50 nm. In a second step we carefully checked the interaction of binary and ternary films with the container material. No reaction is observed at temperatures below 550 C which is supported by the fact that the CIS-films do not adhere to the substrate after thermal treatment in the DSC. Thermal history of the binary and ternary films during and after preparation turned out to have strong effect on the measurement signal as Cu-In, Cu-Se and In-Se react partially even at room temperature. By carefully preparing the specimens we are able to determine reaction temperatures with an accuracy of 1 K and reaction enthalpy with approx. 10 % if the peaks are well resolved. XRD, EDX and SEM techniques are applied for the characterization of completely and partially reacted films. Financial support by the Bayerische Forschungsstiftung is acknowledged.
SESSION G5: II-VI BASED THIN FILMS - B
Chair: Mowafak M. Al-Jassim
Thursday Afternoon, December 4, 1997
1:30 PM G5.1
MODEL FOR EFFECTS OF NA ON POLYCRYSTALLINE CU(IN,GA) DISELENIDE AND ITS SOLAR CELL PERFORMANCE. David Cahen, Leeor Kronik, Dept of Materials and Interfaces, Weizmann Inst of Science, Rehovot, ISRAEL; Hans-Werner Schock, Inst fur Physikalische Elektronik, Univ of Stuttgart, GERMANY.
We present a defect chemical model for the remarkable electronic effects of sodium on thin polycrystalline films of Cu(In,Ga)Se2 (CIGS) and solar cells made with these. The model is based on the oxidation-related passivation of CuInSe2 grain boundaries and on the catalytic effects of alkali metals on semiconductor surface oxidation. Oxidation enhancements of several orders of magnitude have been reported for Na on Si and GaAs. Alkali-metal-induced polarization of the O-O bond of the physisorbed O2 molecule, as well as enhanced tunneling from the substrate due to alkali-metal-induced lowering of the surface work function result in increased formation of (O2)-. The latter dissociates easier into atomic oxygen than the O2 molecule. Hence, a catalytic effect is obtained. In the case of CIGS the probable reaction site is In, rather than Cu, both in view of the thermodynamic driving force for In-O bond formation and because experimentally it is the In oxide that is found on the surface of films and crystals. These catalytic effects of Na are incorporated in our earlier model for CIS oxidation. According to that model, donor defect sites at CIGS surfaces, including grain boundaries, are neutralized by oxide, obtained from molecular oxygen. These donor defect states are detrimental to solar cell performance because they decrease the effective p-type doping of the layer and act as traps for recombination of photogenerated electrons with holes. Both the grain boundary and surface oxidation, as well as Na diffusion processes can be aided by the presence of water, probably through the involvement of hydroxyl species and protons. The proposed model is correlated to chemical and (opto)electronic effects in thin polycrystalline CIGS films and solar cells based on such films by reviewing the experimental evidence available in the literature.
1:45 PM G5.2
Na IMPURITY CHEMISTRY IN PHOTOVOLTAIC CIGS THIN-FILMS MEASURED WITH X-RAY PHOTOELECTRON SPECTROSCOPY. David W. Niles, Kannan Ramanathan, Falah Hasoon, Rommel Noufi, National Renewable Energy Laboratory, Golden, CO; Brian J. Tielsch, Julia E. Fulghum, Chemistry Department, Kent State University, Kent, OH.
Thermal processing of Cu(In1-xGax)Se2 thin-films grown as part of a photovoltaic (PV) device on soda-lime glass leads to the incorporation of Na impurity atoms from the soda-lime glass into the Cu(In1-xGax)Se2. Na increases the photovoltaic efficiency of Cu(In1-xGax)Se2-based devices and has been crucial to the development of record-setting 17.7 efficient PV devices. The purpose of this investigation is to develop a model for the chemistry of the Na in an effort to understand how it improves performance. An analysis of x-ray photoelectron spectroscopy data shows that the Na concentration is 10 atomic percent on the surface and 0.1 atomic percent in the bulk of the CIGS thin-film. Analysis of the Na KLL Auger parameter between the surface and bulk Na shows that the surface Na is chemically different from the bulk Na. Core level binding energies are inconsistent with the presence of NaOH and Na2O in the bulk of the CIGS thin-films, and consistent with a transition from Na2SeO3 on the surface to NaSe in the bulk. The identification of Na-Se bonds in the bulk leads the authors to conjecture that Na must replace In and Ga on the surfaces of Cu(In1-xGax)Se2 grains. Replacement of In and Ga would lead to NaIn or NaGa defects that would act as a double acceptor states. The presence of these defects increases the p-type conductivity of Cu(In1-xGax)Se2 thin-films, consistent with the observed improvement in Cu(In1-xGax)Se2 PV device performance.
2:00 PM G5.3
EFFECTS OF Sb ON THE STRUCTURES AND PROPERTIES OF CuInSe2 HOMOJUNCTION. Gin-Yen Chiang, Geon-Wen Chang and Bae-Heng Tseng, Institute of Materials Science and Engineering, National Sun Yat-Sen University, Taiwan, TAIWAN; Huey-Liang Hwang, Department of Electrical Engineering, National Tsing Hwa University, Taiwan, TAIWAN.
Antimony, a p-type dopant to CuInSe2, tends to segregate on surface during the film deposition. It behaves as a surfactant which modify the surface processes when the film is growing. This promotes two-dimentional growth process and results in a smooth, mirror-like surface. A great improvement in surface morphology is found specifically for Cu-rich film, which has a rough surface when the film grows without Sb. A compact grain structure is also observed in an electron microscope. The film grown with Sb has resistivity as low as 0.01 -cm and a hole mobility higher than that of the film grown without Sb. Subsequent growth of an n-type In-rich film duplicates the structure of the underlying p-type film. Comparing the I-V.......grown with and without Sb, we find that the former has a better junction perfection factor a stronger photovoltaic response. It is attributed to an improvement in the structure in the polycrystalline film and at the junction.
2:15 PM G5.4
LARGE-AREA ZnO FILM GROWTH BY LASER ABLATION FOR PHOTOVOLTAIC APPLICATIONS. Pritish Mukherjee, John Cuff, Alfred M. Miyawa, Robert Jones and Sarath Witanachchi, Laboratory for Advanced Materials Science and Technology (LAMSAT), Department of Physics, University of South Florida, Tampa, Florida.
A dual-laser ablation process has been used to deposit ZnO films on both glass and sapphire substrates. This process produces a high-energy plume that expands rapidly leading to large-area film growth. In comparison to the films grown by conventional single laser ablation, films grown by this technique show a high degree of c-axis alignment on amorphous substrates. The abundance of high-energy species in the plume is responsible for the improved film orientation. Structure of the films grown on glass and sapphire substrates were studied by the x-ray rocking curve technique. Plume energy and expansion leading to large-area deposition depends critically on the coupling of the CO2 laser power into the plasma plume. The effect of the CO2 laser power on the plume expansion was studied by obtaining film thickness profiles at different CO2 laser powers. In addition, enhanced gas-phase reactions in oxygen ambient have been known to promote stoichiometric film growth. A comparison of the optical and electrical properties of ZnO films deposited by single and dual-laser ablation in different ambient oxygen pressures will be presented. Aluminum doped ZnO films from a ZnO:Al target have also been grown by this technique. Optical transmission and carrier mobility of these films will be compared with similar films grown by other techniques.
2:30 PM G5.5
MOVPE AS A TECHNIQUE FOR THE GROWTH OF CdTe/CdS SOLAR CELLS. R. A. Berrigan, S. J. C. Irvine, A. Stafford, Optoelectronic Materials Research Laboratory, North East Wales Institute, Plas Coch, Wrexham, UNITED KINGDOM; D. Ellis and D. J. Cole-Hamilton, Department of Chemistry, University of St. Andrews, St. Andrews , Fife, UNITED KINGDOM.
The CdTe/CdS/(SnO2 or ITO)/glass structure is a promising photovoltaic structure, but reported efficiencies fall well short of the theoretically derived potential of approximately 30%. The major challenge that confronts the field is to promote an enhancement of grain size within the absorbing layer, while retaining control of the doping to produce good junctions for efficient diode structures.
The technique of Metalorganic Vapour Phase Epitaxy (MOVPE) is a useful process for the controlled production of high quality material for electronic device applications. The potential exists to utilise the technique in the deposition, controlled doping and contacting of material suitable for thin, lightweight, highly efficient solar cells suitable for space application.
This work highlights progress made in the MOVPE growth of CdS and CdTe onto ITO coated glass via the controlled pyrolysis of dimethylcadmium, ditertiarybutylsulphide and di-isopropyltelluride. Nucleation studies of CdS onto ITO have revealed a correlation between nucleation behaviour and the properties of the ITO. Temperature is important in the resulting film quality, and kinetic models have been developed to rationalise growth mechanism over a temperature range of 270C to 420C.
The electrical and optical character of the ITO films has been investigated in order to optimise nucleation of CdS onto ITO and produce films exhibiting in SEM images an average grain size of approximately 10m. Absorption and transmission spectra of these CdS films will be presented, indicating that lower temperatures are preferred for MOVPE growth in maximising transmission. Growth of CdTe is shown to produce films also containing grains of 10m average size indicating good nucleation of CdTe onto CdS.
Finally, results will be presented from preliminary doping studies, highlighting the effect of dopants both on material quality and electrical properties of the structure.
2:45 PM G5.6
SEVERAL EFFICIENCY INFLUENCING FACTORS IN CdTe/CdS SOLAR CELLS. K. Li, A.T.S. Wee, J.Y. Lin, K.L. Tan, Department of Physics, National University of Singapore, SINGAPORE; Z.C. Feng, EMCORE Corporation, Somerset, NJ; L. Zhou, S.F.Y. Li, Department of Chemistry, National University of Singapore, SINGAPORE; H.C. Chou, S. Kamra, A. Rohatgi, School of Electrical & Computer Engineering, Georgia Institute of Technology, Atlanta, GA.
Several efficiency influencing factors in MOCVD-grown CdTe/CdS solar cells, including intefracial mixing, preferential crystal orientation of CdTe layers, CdTe grain size and shape, and surface and interface geometrical morphology, are studied. Auger electron spectroscopy (AES) depth profiling is used to demonstrate that different post deposition annealing methods will influence the compositional interface profile. It is shown that interfacial mixing is important to the performance of CdTe/CdS solar cells as it can reduce the number of interfacial states and recombination centers, thus enhancing the performance of the solar cells. Atomic force microscopy (AFM) analysis predicts the relationship between the grain size and cell efficiency, and reveals that the interfacial geometrical morphology has a significant influence on the photovoltaic property of CdTe/CdS solar cells. Smooth interfaces reduce the energy loss due to internal reflectance and increase the photovoltaic conversion efficiency of solar cells. X-ray diffraction shows that polycrystalline CdTe/CdS solar cells with higher efficiencies appear to be orientated with more (111) planes of CdTe parallel to the macro-surface. Further suggestion of increasing the CdTe/CdS solar cell efficiency is discussed.
3:30 PM G5.7
ACCEPTOR DOPANTS IN p-TYPE CdTe. Vello Valdna, Tallinn Technical Univ., Inst. of Materials Technology, Tallinn, ESTONIA.
Thin film CdTe is one of the most important materials for fabrication of large-area cost-effective p-CdTe/n-CdS solar cells. As a rule, CdTe films are heat treated with chlorine fluxes to increase their grain size and improve their electrical and optical properties. Chlorine plays a multiple role in CdTe. At low concentrations chlorine forms shallow acceptor complexes with native defects and due to this increases the p type conductivity of CdTe whereas excess single substitutional chlorine acts as a donor. Often, acceptor codopants are used, acceptor impurities are present in starting material, or acceptor can migrate into CdTe layer from the copper diffused back contact. Also, the solubility of dopants is affected by the excess of component elements and the results can be very contradictive. The aim of this work was to investigate the effect of acceptor codopant on the electrical properties of the chlorine treated CdTe layers. Copper as a deep acceptor dopant to CdTe was used. Annealings of CdTe layers under different partial pressures of either tellurium or cadmium were used.
3:45 PM G5.8
SULFUR DIFFUSION IN POLYCRYSTALLINE THIN-FILM CDTE SOLAR CELLS. M. H. Aslan, R. T. Collins, W. Song, J. Tang, and D. Mao, Department of Physics, Colorado School of Mines, Golden, CO; R. K. Ahrenkiel, National Renewable Energy Lab, Golden, CO.
Polycrystalline thin film CdTe solar cells are a promising candidate for low cost, high efficiency photovoltaic applications. Fabrication of these cells involves deposition of a CdTe absorber layer on top of a CdS layer. The CdS acts both as an n-type contact and a window. Interdiffusion between the CdS and CdTe layers during post growth annealing treatments has been shown to have a dramatic influence on cell performance. In this study, we have used x-ray diffraction (XRD), low-temperature photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE) to characterize the diffusion of sulfur from the CdS layer into the CdTe layer.
Sample preparation involved chemical bath deposition of an 0.2m CdS layer onto tin-oxide coated glass. A CdTe layer 3-5m thick was then electrodeposited onto the CdS. Samples were treated with CdCl2 and subsequently annealed at temperatures up to 450C. Sulfur diffusion into CdTe results in the formation of a CdTe1-xSx ternary phase. For x < 0.2 both the lattice constant and the bandgap of the ternary decrease with increasing S concentration allowing XRD and PL to be used to monitor the S concentration. Depth profiles were obtained by successively etching away layers of the CdTe between measurements. For annealing temperatures of 350C or less, interdiffusion was small (near the detection limit). At 410C , which is the optimal annealing temperature for formation of high efficiency cells in our process, the S concentration varied from x 0.05 at the CdS/CdTe interface to <0.01 at the CdTe surface. Evidence for the presence of both pure CdTe and for the ternary phase was also observed in XRD, PL, and PLE. For anneals at 450C only the ternary phase was present, and the S concentration was fairly uniform across the film with an average x value of approximately 0.08. S diffusion also showed a dependence on the amount of CdCl2 applied to the films prior to annealing. We will discuss the role of grain boundaries in the interdiffusion process and the implications of interdiffusion on cell performance.
4:00 PM G5.9
THE MICROSTRUCTURE AND CHEMISTRY OF THE CdS/CdTe INTERFACE. M.M. Al-Jassim, R.G. Dhere, F. S. Hasoon, K.M. Jones, A. A. Al-Douri, and P. Sheldon, NREL, Golden, CO.
Despite the fact that structural defects in thin-film CdTe act as active electrical recombination sites, and the likelihood of their deleterious effects, little work has been done on the generation and propagation of such defects. In this work detailed TEM examination was carried out on polycrystalline CdTe films to study the nature, origin and three dimensional distribution of such defects. Furthermore, cathodoluminescence examination was used to study the recombination behavior of these defects as a function of deposition conditions and post-deposition treatments. Additionally, in an attempt to study S and Te interdiffusion, fine probe X-ray microanalysis was carried out on the CdS/CdTe interface using a high resolution TEM. The CdTe films were deposited by close-space sublimation (CSS) on CdS/SnO2/Si, single crystal CdS, and single crystal CdTe substrates. Our results showed that the effect of the crystallinity of the underlying substrate on the defect density in the CdTe films is minimal. Under most growth conditions the majority of the defects were generated at the CdS/CdTe interface. Lattice mismatch between the CdS and CdTe, interface irregularities, and the nucleation mode all played a major role in defect generation. The mechanisms by which the defects are generated and the recombination characteristics of these defects will be discussed.
4:15 PM G5.10
EFFECTS OF BACK-CONTACT TREATMENTS ON JUNCTION PHOTOLUMINESCENCE IN CdTe/CdS SOLAR CELLS. D.H. Levi, L.M. Woods, D.S. Albin, R.K. Ahrenkiel, D.H. Rose, D.W. Niles, T.A. Gessert, and P.Sheldon, National Renewable Energy Laboratory, Golden, CO.
Device performance in CdTe/CdS solar cells is critically linked to the quality of the back contact. In this paper we report on a previously unreported phenomenon wherein application of the back contact significantly alters the electro-optical properties of the absorber near the junction. We have studied the photoluminescence spectrum of the near-junction CdTe region in CdTe/CdS solar cells before and after contact application. We have found that the elemental Tellurium layer formed on the CdTe surface by the standard nitric-phosphoric etch process results in a dramatic qualitative change in the junction photoluminescence spectrum. Prior to NP etch, the spectrum has two peaks at energies of 1.50 eV and 1.45 eV, corresponding to recombination in bulk CdTe, and in a CdTeS alloy with 8% sulfur content, respectively. After NP etch, the spectrum consists of a single broad peak at 1.48 eV. We present results of density- and temperature-dependent photoluminescence spectrum and lifetime measurements on a variety of back contact preparations.
4:30 PM G5.11
INTERFACE REACTIONS IN CdS/CdTe SOLAR CELL PROCESSING. Dave Albin, Ramesh Dhere, Doug Rose, Xiaonan Li, Dean Levi, and Peter Sheldon, National Renewable Energy Lab., Golden, CO.
The best performing CdS/CdTe solar cells today typically involve a superstrate structure in which CdTe is grown on a CdS/SnO2/Glass substrate. In the close-spaced-sublimation (CSS) process, high substrate temperatures in the range 550 to 620C are common. Understanding the impact these high processing temperatures have on interfacial reactions between previously deposited layers in addition to the CdS/CdTe interface are important for optimal device performance. The resistance of SnO2 layers to reduction in subsequent thermal anneals is highly dependent upon how the SnO2 layer is processed. Room-temperature sputtered SnO2 shows the least resistance while CVD-grown SnO2 remains stable. Diffusion at the CdS/CdTe interface is also a function of how the CdS is made. Small-grained CdS, resulting from room-temperature processes like chemical-bath deposition (CBD), show more ability to form alloys with CdTe than larger-grained CdS films fabricated at higher temperatures using CSS. Alloy formation is studied by quantum efficiency measurements and glancing-incidence x-ray diffraction (GIXRD) on CdS/CdTe diffusion couples.
4:45 PM G5.12
PREPARATION AND CHARATERIZATION OF CdSxTe1-x ALLOYS FOR CdTe SOLAR CELL APPLICATION. Ramesh Dhere, Doug Rose, Dave Albin, Amy Swartzlander, Sally Asher, Mowafak Al-Jassim and Peter Sheldon, National Renewable Energy Laboratory, Golden, CO.
Interdiffusion at the CdS/CdTe interface significantly effects device properties of CdS/CdTe solar cells. The degree of interdiffusion is mainly influenced by the CdTe deposition parameters and the post-deposition CdCl2 heat treatment. CdCl2 acts as a flux which promotes recrystallization and thus alloy formation near the CdS/CdTe interface. To develop realistic device models, a complete understanding of the compositional profile and a knowledge of the electro-optical properties of the intermixed layer is essential. There is very limited data in the literature on the properties of CdSxTe1-x alloys for different values of x. In this paper, we present the results of CDs/CdTe interface analysis for the as-deposited and CdCl2 heat-treated samples prepared by close-spaced sublimation (CSS). We will describe a variety of approaches investigated for depositing thin CdSxTe1-x alloy films by CSS. Data on the electro-optical and structural characterization of these alloyed layers for various compositions will be presented.
SESSION G6: THIN FILMS - GENERAL
Chair: David W. Niles
Friday Morning, December 5, 1997
8:30 AM G6.1
STUDIES OF THE CHEMICAL BATH DEPOSITION OF CHALCOGENIDE SEMICONDUCTORS FOR PHOTOVOLTAICS. John McAleese, Paul O'Brien, Imperial College of Science, Technology and Medicine, Dept of Chemistry, London, UNITED KINGDOM.
The growth of the wide band gap materials ZnS and (Cd,Zn)S by the cheap and convenient technique of Chemical Bath Deposition remains largely unexplored in terms of the fundamental understanding of the mechansims involved. In this paper, we provide information on the growth of these two materials using SEM, TEM, XPS and PL as well as speciation data which has allowed us to postulate the mechansims involved.
8:45 AM G6.2
DEPOSITION OF ZnTe THIN FILMS BY CLOSE SPACED SUBLIMATION: STRUCTURAL AND ELECTRICAL STUDIES. D.J. Goyal, P.G. Bilurkar, S.K. Thorat and N.V. Mate, EcoSolar Systems (India) Pvt. Ltd., Pune, INDIA.
Zinc telluride has the potential of being a low-cost, environmentally stable, low-resistance and easily manufacturable back contact for Cds/CdTe solar cells. For the first time, close spaced sublimation (CSS) technique is used to deposit thin films of ZnTe. The results are reported in this study. For optimization of the deposition parameters, conducting tin oxide coated float glass substrates are used. Whereas for electrical studies, ZnTe films are deposited on ordinary (soda-lime) float glass.
The effects of substrate temperature and film thickness on the structural properties of the deposited thin films are studied. X-ray diffractograms show that all the films generally exhibit preferred orientation along the  direction. However, the degree of the preferred orientation changes as a function of the film thickness and the substrate temperature. Increase in film thickness reduces the preferential orientation.
The as deposited ZnTe thin films, being that of p-type semiconductor, are highly resistive. In order to effectively use these as contact to Cds/CdTe solar cells, they are made more conductive by doping copper. The doping is effected by dipping the films in alcoholic solution of copper chloride. The extent of doping is controlled by varying the dipping time. Post deposition and post doping annealing is carried out at 200C, for different durations. The resistivity of all the doped films drops drastically in the initial 10 minutes of annealing.
The effects of substrate temperature, film thickness and doping, on the stoichiometry of the films, are studied using atomic absorption spectroscopy.
9:00 AM G6.3
ULTRATHIN INDIUM TIN OXIDE FILMS ON VARIOUS SUBSTRATES BY PULSED LASER DEPOSITION. X.W. Sun, H.S. Kwok, Dept of Electrical and Electronic Engineering, Hong Kong Univ of Science and Technology, Clear Water Bay, Kowloon, HONG KONG; D.H. Kim, LG Electronics, Seoul, SOUTH KOREA.
Indium Tin Oxide(ITO) thin films with low resistivity of 0.1 0.2 m-cm were deposited on various substrates such as YSZ, glass, and ZnO buffered glass by pulsed laser deposition(PLD). High quality crystalline ITO films could be fabricated on YSZ and ZnO buffered glass. These crystalline ITO films should have applications in flat panel displays as both a electrode and substrate. ITO films grown on (100) YSZ had a single (200) orientation with the FWHM of X-ray rocking curve as narrow as 0.08, while ITO films grown on ZnO (0001) buffered glass had an single (222) orientation with the FWHM of X-ray rocking curve of 2.1. Ultrathin ITO films have been fabricated on YSZ and their electrical properties measured. It is showed that the resistivity of these ultrathin ITO films does not change in a temperature range of 10K-300K, which is due to the highly degenerated nature of ITO. ITO films fabricated on ZnO buffered glass was characterized by Hall effect measurement as a function of temperature. These textured films showed relatively higher electron mobility and lower carrier concentration to those ITO films grown on glass. But the resistivity remained almost the same, and remained nearly constant as temperature varied. This indicates that the resistivity of ITO film has no relation with the crystal orientation or the crystalline structure has a weak effect on it. In conclusion, neither phonon scattering nor grain-boundary scattering has remarkable effect on resistivity of ITO films. The resistivity is dominated by impurity scattering in the range of 10K-300K.
9:15 AM G6.4
STUDY OF INDIUM TIN OXIDE FILMS PREPARED BY RF REACTIVE MAGNETRON SPUTTERING AT DIFFERENT SUBSTRATE TEMPERATURE. Li-jian Meng, Departamento de Fisica, Institute Superior de Engenharia do Porto, Rua de Sao Tome, PORTUGAL; M.P. dos Santos, Departamento de Fisica, Universidade do Minho, PORTUGAL.
Indium tin oxide ( ITO ) films were deposited onto the glass substrates at different substrate temperatures ( RT - 500 C ) by rf reactive magnetron sputtering method. The structural, optical and electrical properties of ITO films have been characterized by X-ray diffraction ( XRD ), scanning electron microscopy ( SEM ), optical transmittance and reflectance, sheet resistance and electrical resistivity measurements. The films deposited at room temperature have preferred orientation along the  direction. As the temperature is increased, the (400) difraction peak intensity increases and results in a preferred orientation along  direction for the films prepared at 500 C substrate temperature. It has been shown that the (222) peak intensity has effect on carrier mobility. The film which has low (222) peak intensity has low Hall mobility. The film prepared at 400C substrate temperature has lowest electrical resistivity ( about 3.7 x 10 .cm ).
9:30 AM G6.5
PREPARATION AND PHOTOVOLTAIC PROPERTIES OF TIN SULFIDE AND TIN OXYSULFIDE THIN FILMS BY SPRAY PYROLYSIS TECHNIQUES. Tsuyoshi Kosugi, Kenji Murakami, and Shoji Kaneko, Shizuoka University, Hamamatsu, JAPAN.
Tin sulfide (SnS) which has the bandgap of 1.2-1.4 eV and absorption coefficient over 104 cm-1 in the visible light range, is suitable for absorbing layer of thin film solar cell. Spray pyrolysis technique has the merits of simplicity of its apparatus and good productivity in large area film formation. This is essentially fit for solar cell production. Few results have been reported about SnS solar cell. In this study, we have attempted firstly the preparation and characterization of SnS thin films by using the spray pyrolysis technique. Here, it has been noteworthy that we found a novel compound, tin oxysulftde (Sn2OS) as a promising material for the absorbing layer.
Starting acidic aqueous solutions including tin(II) chloride and thiourea were intermittently atomized with compressed air or nitrogen gas. The SnS thin film was obtained on Corning 7059 glass substrate at relatively high solution concentration or 0.1M and low substrate temperature of 250C. The conditions for obtaining SnS thin films of good quality were relaxed significantly in nitrogen atmosphere. Under the conditions of low solution concentrafion of 0.04M and high substrate temperature of 350C, Sn2OS thin film, which was a semiconductivic material with the bandgap of 1.5 eV, was deposited from the same starting solution described above.
The fabrication of thin film solar cell using SnS or Sn2OS as absorbing layer was performed in which SnO2, ITO, CdS etc. were used as a window layer. All of them showed photovoltaic properties under the condition of AMI.5 solar incidence. Postheating of the above-mentioned solar cells was effective to improve the cell performance. It was, therefore, proved that both SnS and Sn2OS thin films prepare by spray pyrolysis technique have a potential for absorbing layer of thin film solar cell.
9:45 AM G6.6
TIN OXIDE THIN FILMS PRODUCED BY MODIFIED ATMOSPHERIC CVD PROCESS. H. Belis, R.Lewis, R.Condrate, X.W.Wang, Alfred University, Alfred, NY.
Transparent and conductive tin oxide films were deposited on SLS float glass substrates by a modified aerosol CVD system under ambient atmospheric conditions. Sample sizes were between 100 and 400 sq. cm. Precursor solution was tin chloride pentahydrate, and it was excited into an aerosol mist by an ultrasonic nebulizer. The mist was then fed into an atmospheric CVD chamber. Following thin film properties will be discussed: thickness uniformity, resistivity, optical transmission, and morphologyy. Potential applications of this aerosol CVD process will also be discussed.
10:30 AM G6.7
SOLUTION-GROWN CdS THIN FILMS FOR SOLAR CELLS. Jae Heyg Shin, Seung Wook Hyeun, Sung-Ho Shin, Jung Il Park, and Kwang Ja Park, Thin Film Laboratory, National Institute of Technology and Quality, Kyeonggi-do, KOREA.
Thin films of CdS were prepared from an solution containing Cd(Ac)2, NH4OH, NH4Ac and (NH2)2CS for solar cells. Growth rate of CdS films was increased with increasing temperature of reactive solution and with decreasing concentration of NH4OH. Optical transmittances were more than 60%, independent with temperature and concentrations and was changed with thickness of CdS films. Grown films mostly showed the presence of polycrystallincs with mixed cubic and hexaginal phases, but showed the hexagonal preferred phases in some growth condition. The resistivities of CdS were decreased by doping boron from 104 to 101 order ( cm) and the critical amount of dopant was 5X10-3mol %.
10:45 AM G6.8
THE CHARACTERIZATION OF CHEMICAL BATH DEPOSITED CDS ON SINGLE CRYSTAL INP SUBSTRATES. Gerald Riker, M.M. Al-Jassim, and F.S. Hasoon, NREL, Golden, CO.
InP-based solar cells have been attracting increasing attention in both space PV and TPV. For either application there is a great need for a window layer for InP. This will greatly reduce the surface recombination velocity and significantly improve cell performance. In this work we have investigated CdS thin films as possible passivating window layers for InP. The CdS films were deposited on single crystal InP by chemical bath deposition. The film thickness was varied from 200 to 1000‰. The film morphology was investigated by high resolution SEM, while the film microstructure was studied by grazing incidence XRD and cross-sectional TEM. Our investigation showed that the films reach coalescence while the thickness is below 500‰. Furthermore, the CdS films are fine-grained polycrystalline. The grains tend to be smaller close to the CdS/InP interface and grow in size as the film thickness increases. TEM cross-sectional examination and depth profiling revealed the presence of interface contaminant. The effect of such contaminants on the film morphology and microstructure were studied, and various approaches for InP surface cleaning/treatment were investigated. The findings of these studies will be presented and discussed.
11:00 AM G6.9
SPECTRAL UTILIZATION IN THERMOPHOTOVOLTAIC DEVICES. Marvin B. Clevenger, Christopher S. Murray and David R. Riley, Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, West Mifflin, PA.
III-V semiconductor thin film assemblies are being processed into series interconnected devices, known as monolithic interconnected modules (MIMs), for thermophotovoltaic energy conversion. These devices consist of an array of p- and n-doped indium gallium arsenide (InGaAs) layers on a common semi-insulating indium phosphide (InP) substrate. During operation, infrared radiation from an emitter operating at 1500 to 2600F is incident upon the MIMs with a lambertian, energy-weighted distribution. Spectral control strategies have been employed to optimize the conversion of useful infrared radiation into electricity while recuperating the lower energy light. The ratio of the absorbed energy greater than the bandgap to the total energy absorbed in the device is referred to as the spectral utilization. Presented herein are results of spectral utilization measurements for 0.74 eV and 0.60 eV MIMs with several different device architectures.
11:15 AM G6.10
THE PERSPECTIVES OF HIGH-RATE LOW FREQUENCY a-Si:H FILMS DEPOSITION: SOLAR CELL APPLICATION AND STABILITY CONTROL. Boris G. Budaguan, Arcady A. Aivazov, Mark N. Meytin, and Andrei G. Radosel'sky; Moscow Institute of Electronic Technology, Moscow, RUSSIA.
The perspectives of solar cell application of structural inhomogeneous a-Si:H films deposited at high growth rates ( 1 nm/s) from 100% SiH4 in low frequency (55kHz) glow discharge plasma have been investigated. In this case the influence of structural inhomogeneity on dark dc and photoconductivities and light-induced defect generation kinetics (Staebler-Wronski effect, SWE) in a-Si:H films have been studied. The microstructure of films was investigated by IR spectroscopy analysis. Microstructural parameter R=[SiH2]/([SiH]+[SiH2]), was used for the quantitative characterization of structural inhomogeneity in the material bulk. It was found that Fermi level position is fixed by deep defect states and does not depend on parameter R. The comparative analysis of photoconductivity modeling and ESR measurements have shown that recombination in a-Si:H films is controlled by deep D0 states in the material bulk and does not depend on parameter R. Meanwhile it was found that the kinetics of light-induced defect generation is controlled by structural inhomogeneity content. Thus, the above results allow to perform an independent control of stability and electronic properties of a-Si:H films deposited in LF glow discharge plasma.
11:30 AM G6.11
THE PROPERTIES OF a-Si:H/c-Si HETEROSTRUCTURES PREPARED BY 55 kHz PECVD FOR SOLAR SELL APPLICATION. Boris G. Budaguan, Arcady A. Aivazov, Alexei A. Sherchenkov, Andrei V. Biriukov, Institute of Electronic Technology, Moscow, RUSSIA; Vladimir D. Chernomordic, Institute of Microelectronics, Russian Academy of Sciences, Yaroslavl', RUSSIA; J.W. Metselaar, University of Technology, Delft, NETHERLANDS.
The 55 kHz PECVD allows to increase sufficiently hydrogenated amorphous silicon thin film growth rate in comparison with commonly used 13.56 MHz PECVD what is important in many cases. In spite of inhomogenious structure a-Si:H films formed by this method have good electronic properties and high photoconductivity. However, it is important to characterize a-Si:H/c-Si heterostructure from the viewpoint of device application. For this purpose capacitance-voltage and current-voltage characteristics of a-Si:H/c-Si heterostructures were investigated depending on substrate temperature.
The a-Si:H films were deposited with the growth rates of ( 0.5-1.6 nm/s in silane plasma (100% SiH4) at RF power of 200 W and gas pressure 70 Pa). The hydrogenated amorphous silicon layers with the thicknesses of 0.54-1.62 were formed on p-type monocrystalline silicon wafers at substrate temperatures in the range of 40-325 C. Capacitance-voltage characteristics of a-Si:H/c-Si heterostructures were measured at frequencies of 200 Hz, 2 kHz, 20 kHz, 1 MHz. The influence of substrate temperature on surface states density was estimated from the analysis of the results or capacitance-voltage measurements.
Current-voltage characteristics of a-Si:H/c-Si heterostructures were measured at forward and reverse biases in the voltage range of 0-15 V and at different temperatures (from room temperature to 180 C). The temperature dependencies of current-voltage characteristics allowed to determine charge carrier transport mechanizm in a-Si:H/c-Si heterostructures and its correlation with the substrate temperature.
The analysis of the current-voltage and capacitance-voltage characteristics of a-Si:H/c-Si heterostructures showed that a-Si:H films prepared by 55 kHz PECVD is perspective for the device application.
11:45 AM G6.12
REACTION ENERGETICS FOR DESORPTION OF H2 FROM TWO a-Si:H SURFACES. Karland A. Kilian, Dept of Physics, University of Illinois, Urbana, IL; and James B. Adams, Dept of Chemical, Bio, and Materials Engineering, Arizona State University, Tempe, AZ.
We have used the local-orbital density functional code of Sankey and Drabold to model two surfaces of hydrogenated amorphous silicon. The surfaces differ in their H concentrations: one has a very low concentrations, while the other contains 16 at.% H, which is in the range of device-quality material. Both surfaces are completely hydrogen-passivated.
We then study a key component of the growth of a-Si:H by PECVD: H2 desorption from a surface. We create small clusters from our surface models and then apply methods from the Gaussian94 suite of programs to determine the reaction energetics. We do this for several sites on both surfaces and compare the results.