Wide Band Gap Semiconductor Research
Zinc oxide (ZnO) is a very interesting wide band gap semiconductor material, because of its direct band gap, large exciton binding energy, and piezoelectric properties. In addition, ZnO doped with Al is a transparent conductor. By combining its electronic, optical, and electro-mechanic properties, ZnO offers the unique possibility to create multi-functional, integrated devices, that may combine sensing, processing, and actuating functions, in one monolithic structure.
One of the main remaining obstacles, limiting the development of ZnO, is the difficulty in achieving p-type doping of this material, and the lack of detailed understanding about the influence of intrinsic and extrinsic lattice defects on its electronic transport properties. The physics of defects in ZnO is quite complex and to a large extend unknown, and the understanding of doping and conduction mechanisms in this material are still incomplete and challenging. Thorough and systematic studies of the mechanisms of electrical transport, and the effect of the interaction between defects and dopants on these mechanisms, are scarce.
In view of these deficiencies and considering that zinc oxide and its compounds are being considered as important materials for next generation short-wavelength optoelectronic devices, and for future potential applications in multifunctional devices, transparent electronics, and spintronics, we have started a comprehensive study of electronic doping and electrical transport properties of ZnO, and ZnO compounds.
Our research projects in the area of wide band gap semiconductors include:
a) Studies of the effect of heat treatment in various gas atmospheres on the structural and electrical transport properties of thin films of zinc oxide,
b) Investigation of mechanisms of electrical transport in zinc oxide,
c) Exploration of deposition and structural, electrical, and optical properties of thin films of ZnO and ZnS1-xOx grown by sputtering deposition,
d) Studies of p-type doping of single crystal ZnO by conventional ion implantation,
e) Investigation of p-type doping of polycrystalline ZnO by no-conventional ion implantation, and
f) Development of transparent thin film transistors and devices.
a) Studies of the effect of heat treatment in various gas atmospheres on the structural and electrical transport properties of thin films of zinc oxide.
This project was carried out by the student O. Hamad for her M. S. thesis. The work was carried out in collaboration with Dr. Neelkanth Dhere from Florida Solar Energy Center, University of Central Florida, Orlando, FL.
The effects of thermal treatment in O2, air and N2 gas atmospheres, on the electrical transport properties of thin films of ZnO, prepared by sputtering deposition, were investigated. The deposition was performed using an O2 – Ar mixture as sputtering gas, with less than 20% O2 content, resulting is oxygen deficient films (Zn: 51%, O: 49%), with moderately n-type conductivity (Rs: ~ 3x102 Ω/square, n: ~ 8x1014 cm-2, N: 4x1019 cm-3, m: ~ 3x101 cm2/V.s). It was found that heat treatment in any of these atmospheres leads to grain growth (determined by atomic force microscopy), but no significant changes in stoichiometry or thickness (determined by Rutherford backscattering spectrometry). The analysis of the electrical transport properties was based on well-established models for the interpretation of Hall effect and conductivity measurements in polycrystalline materials. Annealing in O2 gas, led to a reduction in carrier concentration, and mobility, presumably due to the incorporation of oxygen into the grains, and the grain boundaries. Annealing in N2 gas led to almost no change in carrier concentration, but significant reduction in mobility, presumably due to nitrogen enhancing further defect creation. These results indicate that the electrical transport properties of ZnO thin films are extremely dependent on deposition conditions, and post-deposition treatments. The information obtained is being used to support ongoing ion implantation studies in ZnO. This work has been published in the journal Thin Solid Films.
The mechanisms of electrical conduction in zinc oxide thin films, grown by pulsed laser deposition, are been investigated as a function of preparation conditions. In a first set of experiments films were deposited on glass and silicon nitride coated silicon, using oxygen rich, oxygen deficient, or nitrogen atmospheres. The substrates were held at 473 K during deposition, and subsequently cooled down to room temperature in oxygen rich, or oxygen deficient atmospheres. Films deposited and cooled in an oxygen deficient atmosphere exhibited very high donor concentration, originated in intrinsic defects, and an impurity band related mechanism of conduction. Films deposited under relatively high oxygen pressure were highly resistive and showed, upon ultraviolet light irradiation, grain boundary controlled electrical transport. An enhancement of the conductivity was observed when using a nitrogen atmosphere during the deposition, and oxygen atmosphere during cooling. In this case, the dependence of the conductivity with temperature followed Motts’ Law of variable range hopping, characteristic of a material with localized states randomly located in space. Since the density of hopping centers appears to be much larger than the density of nitrogen incorporated in this sample, it appears that the nitrogen induces defects in the zinc oxide lattice that behave as localized hopping centers, as well as carrier suppliers, giving rise to the observed conductivity. Part of these studies have been presented at the 2004 Fall meeting of the Materials Research Society and published in the Proceedings of the meeting. A more comprehensive description of the work has been recently submitted for publication in the journal Thin Solid Films.
c) Exploration of deposition and structural, electrical, and optical properties of thin films of ZnO and ZnS1-xOx grown by sputtering deposition.
We are studying the structural, electronic, and electrical transport properties of thin films of ZnO and ZnS1-xOx prepared by dual-beam radio-frequency (RF) sputtering deposition. It is expected that the addition of S to ZnO will result in an increase in band gap and a movement of the conduction band edge away from shallow donors, thus increasing the activation energy of these centers. The increase in band gap may be useful for multilayer structures and band gap tuning, and the increase in activation energy of residual donor states should facilitate p-type doping (by reducing intrinsic n-type conductivity). This project is in progress. Several samples have been prepared already. The composition has been studied by Rutherford backscattering spectrometry, the band gap has been determined by spectrophotometry (in transmission mode), and the electronic transport properties by Hall effect and conductivity measurements. X-ray diffraction will be used in the near future to determine the phases formed. It is too early at this point to draw any conclusions from these studies.
d) Studies of p-type doping of single crystal ZnO by conventional ion implantation.
This project is carried out in collaboration with Dr. Michael Lorenz from Universität Leipzig, Institut für Experimentelle Physik II, D-04103 Leipzig, Germany, and Dr. Gerhard Brauer from Forschungszentrum Rossendorf, D-01314 Dresden, Germany.
Single crystal samples of ZnO have been implanted with nitrogen ions having energies of
35 – 40 keV, to doses of 2x1014 – 1x1015 cm-2, with the substrate held at room temperature. Samples were subsequently annealed at temperatures ranging from 300 °C to 1100 °C, in air. Ion channeling measurements of as-implanted samples show that there is significant defect recombination during implantation. Annealing at 1100 °C results in channeling spectra similar to those of the unimplanted samples, suggesting complete removal of implantation induced defects. The temperature dependence of the integral n-type carrier concentration, measured by Hall effect, seems to indicate a lower level of residual donor concentration for samples annealed at higher temperature. Detailed low temperature (2 K), and temperature dependent photoluminescence measurements, reveal the formation of acceptor centers in samples annealed at 700 °C. Further studies are in progress including ion implantation with N+, P+, and As+ ions, and characterization of vacancy type defects by means of positron annihilation analysis, as well as channeling, secondary ion mass spectrometry, Hall effect, photoluminescence, and deep level transient spectroscopy studies. This is an ongoing project and we expect to start publishing the results in the near future. We have presented selected aspects of the work at the12th International Workshop on Oxide Electronics that took place in Cape Cod, MA, in October of 2005.
e) Investigation of p-type doping of polycrystalline ZnO by no-conventional ion implantation.
This project is carried out in collaboration with Dr. Vladimir Richter, and Professor Rafi Kalish from Solid State Institute and Department of Physics, Technion, Haifa 32000, Israel.
We are one of the first groups in the world to demonstrate p-type doping of polycrystalline ZnO thin films, by ion implantation with As. After observing the failure of several research groups to obtain p-type doping of ZnO by conventional ion implantation, a novel approach was envisaged. The approach utilized consisted in carrying out the implantations at liquid nitrogen temperature (~ -196 ºC) to freeze the interstitials created by the irradiation, and thus avoid the creation of a defect imbalance that precludes the proper annealing of the implantation-induced lattice damage in ZnO. The low temperature implantation was followed by a rapid in-situ heating of the sample, at 560 ºC, for 10 minutes, to induce short range, interstitial-vacancy recombination, and substitutional lattice location of the dopants. Finally, an ex-situ annealing was carried out at 900 ºC, for 45 minutes, in flowing oxygen, to further anneal any residual lattice defects, and enhance the electrical activation of the dopants. A hole concentration of 2.5x1013 cm-2 was obtained using this approach, upon implantation of 150 keV, 5x1014 As/cm2. A conventional room temperature implantation of 1x1015 As/cm2, followed by the same annealing, resulted in n-type conductivity with carrier concentration of 1.7x1012 cm-2. Implantation of N+ and P+ ions did not result in p-type conductivity (as determined from Hall effect). The results of this investigation have been published in the journal Applied Physics Letters.
f) Transparent thin film transistors and devices.
In addition to the ongoing research just described, we are performing exploratory experiments with the intention of developing a new research project, devoted to the study of transparent electronic devices, in collaboration with Dr. Neelkanth Dhere from the Florida Solar Energy Center at UCF, and Dr. Santosh Kurinec, Head of the Department of Microelectronics at Rochester Institute of Technology, in Rochester, NY.