"Photovoltaics are the future energy source. We develop
materials science of chalcogenides,
the most promising class of materials, grow single crystals,
do advanced characterization, and develop models to drive advances."

Angus Rockett

Angus Rockett
Professor of Materials Science and Engineering

Office 1-107 Engineering Sciences Building

Telephone 217-333-0417 Fax 217-333-2736

Mail Address Department of Materials Science and Engineering
1304 W. Green St., Urbana, IL 61801

arockett@illinois.edu    Rockett research group page

  • Profile
  • Research
  • Publications
  • Awards

Profile

Angus Rockett received a B.S. in Physics from Brown University in 1980 and a Ph.D. from the University of Illinois in 1986. Dr. Rockett is a Professor of Materials Science at the University of Illinois. His research interests include sputter deposition and characterization of CuInSe 2 for photovoltaics; growth of thin films; microchemical and microstructural analysis of thin films including by TEM, XPS, SIMS, and other methods; modeling of materials, especially semiconductors, using Monte Carlo, density functional theory, and continuum methods; and microelectromechanical systems. Dr. Rockett has more than 115 publications on these topics as well as on dopant segregation during crystal growth, ion source design, and transition metal oxide and nitride deposition and characterization. He has presented more than 40 invited talks.

Dr. Rockett is a Fellow of the American Vacuum Society. He was an Assistant Dean of Academic Programs in the College of Engineering in 1993-94 and in 2000 was a member of the technical staff at the U.S. Department of Energy Office of Basic Energy Sciences. He currently serves on advisory panels for the University Grants Committee of the Government of Hong Kong. Dr. Rockett has participated in the planning of numerous international conferences for the AVS, IUVSTA, TMS and MRS, and serves on several committees of the AVS including the International Interactions Committee. He is a short course instructor for the AVS and has given tutorial lectures for the IEEE, at the Argonne National Laboratory, MRS and at national meetings of the Chinese, Swedish and Mexican Vacuum Societies. He is a member of the Editorial Board of the Journal Materials Chemistry and Physics, the Journal of Vacuum Science and Technology, and has served as a guest editor of several conference proceedings including the Thin Films portion of IVC-12/ICSS-8 and E-MRS 2002, and ICTF-13.

Surface Studies on CIS

Cu(In,Ga)Se2 (CIGS) are promising materials for thin film photovoltaic applications. This work studies the epitaxial growth of CIGS single crystal films on GaAs substrates of various orientations and characterizes the properties of the thin films. A surprising finding is the strong tendency of film surfaces to facet to {112} planes. The work attempted to establish the connections between the film morphology, the surface energies, the surface chemical compositions, and the reconstruction of polar surfaces. Using angle-resolved photoelectron emission spectroscopy, I found that there is a severe Cu depletion at the first 1–2 layer of the free surface of CuInSe2 and the surface is semiconducting. The results strongly support the model of a reconstructed non-stoichiometric polar surface and exclude the previously believed existence of a bulk second phase on the CIS surface. Unique features of the film morphology suggest that the properties and structure of the polar surfaces have great effects on the growth of the crystals, and probably on the incorporation of the large amount of point defects. Measured chemical composition profiles indicate that the Cu depletion observed on free CIGS surface remains at the CIGS/CdS heterojunction interface and Cd is incorporated into the surface of CIGS. It is proposed that this non-stoichiometric composition leads to charge imbalance at the interface and causes the type-inversion of the CIGS surface, which are favorable for the device performance.

Study of CIS Grain Boundaries

Photovoltaics based on CuInSe2 and related materials have the highest performance of any thin film devices. However, many questions remain to be answered concerning their operation.  In particular the effect of grain boundaries on performance is of great interest.  The devices typically consist of polycrystalline p-type Cu(In,Ga)(S,Se)2 (CIGS) semiconductor alloys deposited on Mo-coated soda-lime glass substrates.  The CIGS is coated with intrinsic CdS by a chemical bath deposition method.  Finally, a top n-type transparent conducting oxide contact is applied.  The devices are generally thought to be limited by recombination of carriers in the space-charge region, although the details of defects involved in this recombination and the properties of the CIGS/CdS heterojunction are still very unclear.  In addition to polycrystalline CIGS devices, a limited number of devices have been produced using single-crystal epitaxial layers of CIGS on GaAs.  The performance of these devices is generally found to be inferior to that of the polycrystals, in spite of controlled variation in surface orientation and surface polarity based on selection of the GaAs substrate orientation.  The lower performance of the single crystal devices is probably not due to less optimization of the process conditions and is likely primarily the result of the grain boundaries themselves.  Two major questions must be resolved to understand this remarkable result, first how the grain boundaries can fail to have a detrimental effect on the device by mediating carrier recombination, as is typical in other semiconductors, and second what benefits they provide that are not available in single crystals.

Amorphous CdGeAs2 for detectors

The goal of this project is to develop new materials for high resolution, room temperature gamma radiation detection. We propose to develop high Z, high resistivity, amorphous, semiconductors to be used as solid-state detectors at near ambient temperatures.  The principle of operation will be analogous to single crystal semi-conducting detectors (e.g. Ge or CdZnTe) which are designed to exploit the electron generation aspects of gamma ray interactions with matter (e.g. photo-electric effect, Compton scattering, and pair production).

These new materials must have two important properties: 1) maximize the number of electrons produced from gamma events and 2) efficiently collect those electrons as the measurable response signal.  The first property requires materials with high attenuation coefficients for gamma interaction.  This can be roughly correlated to the atomic number, and thus is strongly compositionally dependant.  The second property for efficient collection of electrons produced from gamma interactions is strongly correlated to the electronic structure of the material.  This can be achieved by using highly biased, high resistivity, semiconductors, such that any electrons produced from a gamma event are rapidly swept across the band gap into the conduction band and collected.

Growth of Cu(InGa)SE2

The typical layer structure of a current high-efficiency CuInGaSe2 (CIGS) device is as follows: a soda-lime glass substrate, Mo back-contact, CIGS absorber layer (p type), CdS buffer layer (n−), and ZnO (n++) window layer. In a typical device, the CIGS absorber layer is a polycrystalline film of 1.5–2 micrometer thickness. The film is deposited on a Mo-coated soda-lime glass in one of the many multi-stage processes, either by coevaporation or by selenization of metallic precursor films. On top of the CIGS absorber, a thin (»60 nm) CdS buffer layer is grown by chemical bath deposition (CBD), followed by Al-doped ZnO window layer deposited by sputtering or chemical vapor deposition. With evaporated fingershaped Al electrode on ZnO, the device is completed. The photovoltaic cell generates electromotive force through the separation of photo-generated positive and negative charges by a potential barrier. In the CIGS device, the ZnO/CdS/CIGS heterojunction provides the potential barrier. Most of the light is absorbed in the p-type CIGS layer (there is some absorption of radiation above 2.4 eV in the thin CdS layer), and photo-generated electrons are swept into the n type region once they migrate into the built-in field of the heterojunction.

CIGS films in our lab are deposited using a hybrid sputtering and evaporation technique. Commercially supplied “epi-ready” (110), (001) and (111)B and (111)A GaAs wafers are used as substrates. The In, Ga, and Cu fluxes are generated by magnetron sputtering of In and Cu or Cu-Ga alloy targets in 99.9999% pure Ar at 0.3Pa. Se is supplied in excess from an effusion cell. Layers with a range of compositions can be obtained by varying the flux ratio of Cu-Ga relative to In and by changing the Cu-to-Ga ratio in the Cu-Ga target. Growth temperatures range from 480 ±C to 720 ±C. Typical growth rates are 1.0 micron/h and the thickness of grown layers ranges from 0.1 microns to 2 microns. Research

Publications

D. M. Diatezua, Z. Wang, D. Park, Z. Chen, A. Rockett, and H. Morkoc, "Si3N4 on GaAs by Direct Electron Cyclotron Resonance Plasma-Assisted Nitridation of Si Layer in Si/GaAs Structure," J. Vac. Sci. Technol. B 16, no. 2 (1998): 507.

David J. Schroeder, Jose Luis Hernandez, Gene D. Berry and Angus A. Rockett, "Hole Transport and Doping States in Epitaxial CuIn1-xGaxSe2," J. Appl. Phys. 83, no. 3 (1998): 1519.

M. Bodegard, K. Granath, L. Stolt, and A. Rockett, "The Behavior of Na Implanted Into Mo Thin Films During Annealing," Solar Energy Mater. and Solar Cells 58, no. 2 (1999): 199-208.

A. Rockett, K. Granath, S. Asher, M. M. Al Jassim, F. Hasoon, R. Matson, B. Basol, V. Kapur, J. S. Britt, T. Gillespie and C. Marshall, "Na Incorporation in Mo and CuInSe2 from Production Processes," Solar Energy Materials and Solar Cells 59, no. 3 (1999): 255-64.

Z. Wang, D. M. Diatezua, D-G. Park, Z. Chen, H. Morkoc, and A. Rockett, "Plasma nitridation of thing Si layers for GaAx dielectrics," J. of Vac. Sci. & Tech. B 17, no. 5 (1999): 2034-9.

A. Rockett, J.S. Britt, T. Gillespie, C. Marshall, M.M. Al Jassim, F. Hasoon, R. Matson, B. Basol, "Na in selenized Cu(In,Ga)Se2 on Na-containing and Na-free glasses: distribution, grain structure, and device performances," Thin Solid Films 372, no. 1-2 (2000): 212-17.

A. Rockett, R. N. Bhattacharya, C. Eberspacher, V. Kapur, and S.H. Wei, "Basic Research Opportunities in Cu-Chalcopyrite Photovoltaics," in Photovoltaics for the 21st Century. Proceedings of the International Symposium. (Electrochem. Soc. Proceedings Vol.99-11). Electrochem. Soc. (1999): 232-40, Pennington, NJ, USA.

D. Liao and A. Rockett, "Epitaxial growth of Cu(In,Ga)Se2 on GaAs(110)," J Appl. Phys 91, no. 4 (2002): 1978-83.

Y.M. Strzhemechny, P.E. Smith, S.T. Bradley, D.X. Liao, A.A. Rockett, K. Ramanathan, and L.J. Brillson, “Near-surface electronic defects and morphology of CuIn 1-xGa xSe2,” J.Vac. Sci. & Tech. B 20, no. 6 (2002): 2441-8.

S. Kodambaka, V. Petrova, S.V. Khare, D. Gall, A. Rockett, I. Petrov, and J.E. Greene, “Size-dependent detachment-limited decay kinetics of two-dimensional TiN islands on TiN(111),” Phys. Rev. Lett. 89, no. 17 (2002): 176102/1-4.

J.T. Heath, J.D. Cohen, W.N. Shafarman, D.X. Liao, and A.A. Rockett, “Effect of Ga content on defect states in CuIn 1-xGa xSe 2 photovoltaic devices,” Appl. Phys. Lett. 80, no. 24 (2002): 4540-2.

A. Rockett, D. Liao, J.T. Heath, .D. Cohen, Y. M. Strzhemechny, L. J. Brillson, K. Ramanathan, and W.N. Shafarman, “Near-surface Defect Distributions in Cu(In,Ga)Se2,” Thin Solid Films (2002): 431-2, 301-6.

D. Liao and A. Rockett, “Cd-doping at the CuInSe 2/CdS heterojunction,” J. Appl. Phys. 93, no. 11 (2003): 9380-2.

D. Liao and A. Rockett, “Cu depletion at the CuInSe 2 Surface,” Appl. Phys. Lett. 82, no. 17 (2003): 2829-31.

O. Lundberg, J. Lu, A. Rockett, M. Edoff, L. Stolt, “Diffusion of Indium and Gallium in Cu(In,Ga)Se 2 Thin Film Solar Cells,” J. Phys. and Chem. of Solids 64, no. 9,10 (2003): 1499-1504.

A. Rockett, D. Liao, J.T. Heath, J.D. Cohen, Y.M. Strzhemechny, L.J. Brillson, K. Ramanathan, and W.N. Shafarman, “Near-surface Defect Distributions in Cu(In,Ga)Se 2,” Thin Solid Films (2003): 431-2, 301-6.

A. Rockett, DD. Johnson, S.V. Khare, B.R. Tuttle, “Prediction of Dopant Ionization Energies in Silicon: The Importance of Strain,” Phys. Rev. B. 6823, no. 23 (2003): 3208.

A. Rockett, D. Liao, C. Lei, C. Mueller, and I. Robertson, “The Effect of Na in Polycrystalline and Single Crystal CuIn 1-xGa xSe 2,” Thin Solid Films (2005): 480-1, 2-7.

L.C. Yang, G.S. Chen, and A. Rockett, “Surface Polarities of Sputtered Epitaxial CuIn Se2 and Cu1 In3 Se5 Thin Films Grown on GaAs (001) Substrates,” Appl. Phys. Lett. 86, no. 20 (2005): 1-3.

Awards
  • Xerox Award for Faculty Research (1992)
  • Everitt Teaching Award, UIUC (1993)
  • Outstanding Undergraduate Advisor, UIUC (1997-2000)
  • Invited Opponent, Ph.D. thesis defense, Uppsala University (1997)
  • Fellow, American Vacuum Society (1998)
  • Donald Burnett Teacher of the Year Award (1998)
  • Editorial Board, Materials Chemistry and Physics (1999)
  • Stanley Pierce Award, UIUC (2002)
  • Accenture Engineering Council Award for Excellence in Advising (2007)