ICMCTF2003 Session C6: P-type and n-type Semiconducting and TCO Films
Thursday, May 1, 2003 1:30 PM in Room Sunset
C6-1 Application of Novel Compositional-spread Technique to the Synthesis of Optical Thin Films
H.M. Christen, G.E. Jellison, I. Ohkubo, H.Y. Zhai, H.N. Lee, D.H. Lowndes (Oak Ridge National Laboratory)
Continuous compositional-spread (CCS) techniques allow for a multitude of thin-film samples to be synthesized under perfectly identical conditions and with important time savings. This is particularly useful for alloy-type materials whose properties depend critically on the composition. Examples include transparent conducting oxides, ferroelectrics, and electro-optic materials. Unfortunately, previous CCS approaches were subject to serious limitations, including film thickness variations, small sample sizes, restriction to equilibrium-route synthesis, etc. Our new pulsed laser deposition (PLD) based approach overcomes most of these difficulties. In this technique, alloying is accomplished by the repeated sequential deposition of sub-monolayer amounts of the constituents at the elevated film growth temperature. In this presentation, the basic principles and capabilities of the approach are described, and applications to various materials are shown. In particular, we explore the case of SrxBa1-xNb2O6 and show data obtained by electrical and optical characterization techniques.
C6-3 Influence of Processing Parameters on Conductivity in Sputtered Spinel Films
G.J. Exarhos, C.F. Windisch, Jr. (Pacific Northwest National Laboratory); R.R. Owings (University of Florida)
A polaron charge transport mechanism has been confirmed in mixed transition metal spinel oxide films by means of spectroscopic and electrical property measurements as a function of composition and temperature. Relatively low resistivity (milliohm-cm) and long wavelength transparency are found in films sputter deposited under oxidizing conditions from metal alloy or mixed metal oxide targets that contain combinations of nickel, cobalt, palladium, or rhodium. While films comprised of the spinel crystalline phase showed low resistivity, certain amorphous compositions containing nickel and rhodium exhibited even lower resistivity and better transparency than their crystalline analogs. Electronic structure simulations further suggest that spinel oxide compositions containing non first row transition metal cations should exhibit even higher transparency at longer wavelengths. The nature of the substrate on which films are deposited, sputter deposition parameters, and post deposition annealing in air are all found to influence film properties. Results suggest the importance of controlling metal cation oxidation state and grain morphology to optimize conductivity and transparency. Possible application of such p-type conductors as hole injection layers in OLED devices also will be discussed.
C6-4 Characteristics of Indium Oxide Doped with Refractory Metals
Y. Yoshida (Colorado School of Mines); T. Gessert, D. Young, T. Coutts (National Renewable Energy Laboratory)
We report our preliminary work on the properties of indium oxide (IO) doped with refractory metals such as molybdenum (IMO), zirconium (IO:Zr), and titanium (IO:Ti). As is well known, transparent conducting oxides (TCOs) require high mobility to demonstrate good electrical and optical properties. In recent studies, Meng et al. (2001) reported obtaining exceptional mobilities of > 100 cm2/Vs using thermally evaporated IMO. Similarly, Groth et al. (1966) used spray pyrolysis of IO:Zr and IO:Ti and reported mobilities of 170 and 120 cm2/Vs, respectively. In this study, the more industrially relevant technique of r.f. sputtering was used, and functionality of dopants, oxygen-to-argon ratio, and substrate temperature were studied. To date, maximum mobilities attained for IMO, IO, IO:Zr, and IO:Ti are 45, 35, 22, and 12 cm2/Vs, respectively. To investigate the origin of the reasons for the different mobilities, the method of four coefficients (M4C) was used (Young, 2000). M4C measures conductivity, Hall, Seebeck, and Nernst coefficients over a wide range of temperatures to produce direct measurements of the aa film's density-of-states effective mass (m*), the Fermi energy level, and an energy-dependent scattering parameter that is related to a relaxation time (τ), where m* and τ are the parameters that fix the value of mobility. Preliminary results for IMO showed m* and τ of 0.28±0.04 me and 3.63x10-15 seconds, respectively, where me is the mass of the free electron. Considering the consistency of our data to the values reported in the literature for indium oxide of ~ 0.3 me (Ohhata, 1979), a longer τ is the possible cause of the higher mobilities for IMO. This M4C will be applied to the other TCO films under investigation to study if the values of m* and τ vary appreciably with dopants and deposition conditions.
C6-5 Improving the Conductance of ZnO Thin Films by Doping with Ti
Y.-M. Lu (Kun Shan University of Technology, Taiwan, ROC)
Highly conductive and transparent impurties-doped zinc oxide thin films have recently gained much attention because they are composed of inexpensive, abundant materials. The Ti doped ZnO thin films were deposited by simultaneously magnetron co-sputtering from both Zn and Ti targets in a mixture of oxygen and argon gases onto heated Corning 7059 glass substrates.By adjusting the Ar/O2 ratio and other process parameters including RF power, and substrate temperature, the electrical property of ZnO thin films change from an isolation to a good conduction. The results show that deposition rate is an approximately linear function of DC power of Ti target except at 300 watts. At 300watts, the growth rate decreases may due to strong interference between zinc and titanium sputtered atomic fluxes.The incorporation of titanium atoms into zinc oxide films is not effective until the Ti target power increased to a value of 250watts.The atomic percents of titanium in the films are measured to be 1.33% and 2.51% correspond to 250watts and 300watts of Ti target power applied respectively. The XRD patterns shown only a single ZnO phase existing and shifted to lower 2 theta values imply Ti atoms incorporated into the ZnO lattice and occupy the zinc atoms lattice sites.The reistivity of undoped ZnO films is extremely high and decrease to a value of 3.78 - 10-2 ohm-cm when 2.51% atomic percent of Ti is incorporated. All of the zinc oxide films show good transmittance in the range of 4000-7000 A.The average transmittance is 70-80% in this study. The optical energy gap increases with increasing the doping amount of Ti in the films. The maximum value of optical energy gap gained in this study is 3.18 eV when the doping amount of Ti is 2.5 atomic %.
C6-6 Electro-optical and Structural Properties of Thin ZnO Films, Prepared by Filtered Vacuum Arc Deposition
T. David, S. Goldsmith, R.L. Boxman (Tel-Aviv University, Israel)
Thin ZnO films were deposited at room temperature on glass substrates by a filtered vacuum arc deposition (FVAD) system. The electrical, optical and structural properties were investigated as a function of the oxygen pressure in the range of 0.26-0.73 Pa and the arc current in the range of 100 - 300 A. No additional treatment was applied to the samples. Film thickness was in the range of 100 - 400 nm, depending linearly on the arc current. As-deposited resitivity was in the range of (1-2)x10-2 Ω m and the optical transmission of 300 nm thick films was in the range of 85-90% in the visible and near-IR spectral region. Minimal resistivity of 1.05x10-2 Ω m was reached at oxygen pressure of 0.42 Pa, for a 240nm thick film, that had ~94% transmittance in the visible and near-IR range. The relative standard deviation of the measured parameters, determined on a set of seven samples deposited with the same current and pressure was less than 4%. XPS analysis showed that the films were stoichiometric both on the surface and within the film, and that the composition was pressure independant. XRD analysis showed the films to be crystalline, with pressure dependant preferred orientation. In addition, a series of films were prepared in which the substrate holder served as an auxiliary anode. Previous deposition of SnO2 thin films with such an auxiliary anode showed that the film electrical conductivity improved as a function of the current to the auxiliary electrode. It was found that in the present study the film's electro-optical properties were independent of the substrate holder current in the range of 0-6A.
C6-7 Growth Behavior and Optical Properties of Metal-nanoparticle Dispersed Dielectric Thin Films Formed by Alternate Sputtering
K.S. Lee, T.S. Lee, B. Cheong (Korea Institute of Science and Technology, South Korea); S.H. Cho (Ajou University, South Korea); D.Y. Ku (Hankuk Aviation University, South Korea); W.M. Kim (Korea Institute of Science and Technology, South Korea)
Volmer-Weber type island growth mode was adopted to form nano-sized metal particles embedded in dielectric matrix by alternate deposition of metal and dielectric layers. Particle size and volume fraction were controlled by changing the nominal thickness of metal layer and its ratio to that of dielectric layer, respectively. Deposition conditions such as substrate temperature was also varied to modify the spatial distribution of nanoparticles. Plan-view and cross sectional TEM experiments exhibits that the metal particles are spherical in shape and proportional in somewhat complex manner to the nominal thickness of metal layer in size. Since the metal nanocomposites are attractive for potential applications in nonlinear optical devices based on planar waveguide, understanding of propagation properties is very important. Optical loss in guided wave was measured using prism coupler and effect of absorption and rayleigh scattering was estimated as a function of particle size, spatial distribution, surface roughness, etc. As an effort of improving device performance, we used highly polarizable ferroelectrics as medium materials as well as silica glass and investigated optical properties near surface plasmon resonance.
C6-8 Wide Gap P-type Semiconductors, LaCuOS1-xSex Epitaxial Layers
H. Hiramatsu (Hosono Transparent Electro-Active Materials Project, Japan); K. Ueda (Tokyo Institute of Technology, Japan); H. Ohta, M. Hirano (Hosono Transparent Electro-Active Materials Project, Japan); T. Kamiya, H. Hosono (Tokyo Institute of Technology, Japan)
Wide gap p-type semiconductors are vital for fabrication of p-n junction devices such as ultraviolet light emitting diodes (LED). We have reported several p-type electrical conducting wide gap oxides such as CuMO2 and SrCu2O2. P-type conductivity has been explored by developing a guiding principle on the basis of chemical bonding structure of the valence band maximum in oxides. We have extended this idea to oxychalcogenides, leading to the finding of LaCuOCh (Ch = S and Se) with a layered structure exhibits p-type electrical conduction. These materials are direct-transition semiconductors and exhibit excitonic photoluminescence (PL), neither of which have been observed for other p-type wide gap oxides reported to date. In addition, high conductive p-type thin films are required to fabricate LEDs utilizing the excitonic PL properties. In this paper, epitaxial films of LaCuOSxSe1-x were grown on MgO (001) substrates by a reactive solid phase epitaxy (R-SPE) technique. The films exhibited blue-ultraviolet photoluminescence originating from room temperature exciton in each composition. Both p-type electrical conductivity and Hall mobility increased with an increase in the Se content, indicating the enhancement of bond-covalency at the top of the valence band. A Hall mobility of 8.0 cm2V-1s-1, which is as large as that of p-type GaN:Mg, was obtained for LaCuOSe films. Mg2+ doping into La3+ sites led to a conspicuous increase in the hole concentration in LaCuOSe, with keeping a mobility as large as 4.0 cm2V-1s-1. As a consequence, a degenerated p-type electrical conduction was realized with a large conductivity of 141 Scm-1 at a hole concentration of 2.2x1020 cm-3. These features make the LaCuOSe film very promising for such applications as an active layer or p(+) layer of UV-LED and a transparent p-electrode in an organic LED.
C6-10 Transparent and Conductive Nano-laminates of ZnO/Al
J.K. Lin, J.M. Ting (National Cheng Kung University, Taiwan, ROC)
Transparent zinc oxide thin film can be obtained using a sputter deposition technique. The electrical conductivity of zinc oxide thin film is often limited if the thin film is deposited at low temperatures. However, it is desirable to deposit zinc oxide thin films at or near room temperature when certain substrates such as plastics are used. Improvement of the electrical conductivity for room-temperature deposited zinc oxide thin films is thus called for. In this paper, we have examined nano-laminates of alternating ZnO and Al films to show the enhancement of electrical conductivity. In this structure, Al is an ultrathin film with good optical transmittance (> 85%) and low electrical resistivity. The ZnO/Al nano-laminates were deposited using an r.f. magnetron sputter deposition technique at room temperature. The resulting ZnO/Al nano-laminates were characterized for thickness and uniformity using both nanospec and ellipsometry, electrical resistance using a four-point probe method, optical properties using UV-Vis spectrometry, crystallinity using grazing-angle x-ray diffractometry, composition using secondary ion mass spectrometer (SIMS) and Rutherford backscattering spectrometry (RBS), and interface nanostructure examined using transmission electron microscopy (TEM).