ICMCTF2013 Session C2-1: Fundamentals of Thin Films towards Optoelectronics Devices
Tuesday, April 30, 2013 8:00 AM in Room Sunset
C2-1-1 Carrier Transport and Photoluminescence Properties of Ga-Doped ZnO Films Grown by Ion-Plating and by Atmospheric-Pressure CVD
Tomoaki Terasako, Yoshinori Ogura, Shohei Fujimoto (Graduate School of Science and Engineering, Ehime University, Japan); Huaping Song, Hisao Makino (Kochi University of Technology, Japan); Masakazu Yagi (Kagawa National College of Technology, Japan); Sho Shirakata (Graduate School of Science and Engineering, Ehime University, Japan); Tetsuya Yamamoto (Kochi University of Technology, Japan)
Polycrystalline Ga-doped zinc oxide (GZO) films have high visible transmission compared with ITO (Sn-doped In2O3) films widely used for transparent electrodes in optoelectronic devices. We have had an issue to be resolved for GZO films: their electrical resistivity (ρ) is higher than that of ITO films. In this study, we have investigated what determines electrical properties of GZO films. In our previous work, for GZO films with carrier concentration (n) from 3×1018 to 1×1021cm-3 deposited by ion plating with dc-arc discharge (IP), we reported that the grain boundary scattering mechanism plays a minor role in carrier transport. Temperature (T)-dependent Hall mobility (μ) measurements of the GZO films showed a continuous transition in dominant scattering mechanisms in intra grain from ionized impurity scattering mechanism (from non-degenerate (3×1018<n<4×1019cm-3) to degenerate (4×1019<n<3×1020cm-3)) to thermal lattice vibration scattering mechanism (n>3×1020cm-3)) with increasing n.
In this study, to address routes toward better understanding of factors limiting carrier transport of GZO films, a comparison of effects of Ga-doping on electrical characteristics between GZO films deposited by IP (IP-GZO) and those grown by atmospheric-pressure CVD (AP-CVD GZO) has been made. 200-nm-thick IP-GZO films were deposited on glass substrates at a substrate temperature (TS) of 200 ºC. GZO films with the thicknesses of 1-5 μm were prepared on r-plane sapphire substrates at TS =550-750 °C by the AP-CVD using Zn, H2O and GaCl3.
T-dependent Hall-effect measurements showed that the dominant scattering mechanism in intra grain, which depends on n, of AP-CVD GZO is same as that of IP-GZO. Note that the gradient of T-μ curve (Δμ/ΔT) for AP-CVD GZO with n=1×1021cm-3 was a negative value of -0.045, which is about two times of that of IP-GZO with the same n.
PL measurements at 10 K for the AP-CVD GZO films revealed the shift of the dominant near-band-edge (NBE) emission from 3.368 eV at n~2×1018 cm-3 to 3.376 eV at n=4.4×1020 cm-3 accompanied with its asymmetric broadening. This behavior is probably due to a decrease in the exciton binding energy for Ga-related bound exciton line due to the screening of the Coulomb interaction caused by an increase in n. On the other hand, the dominant NBE emission of the IP-GZO film shifted from 3.347 eV at n=3×1018 cm-3 to 3.365 eV at n=8×1019 cm-3. With further increasing n, it seems that the dominant NBE emission of IP-GZO approach that of AP-CVD GZO. We will discuss a comprehensive picture what caused the difference in the photon energy at any given n between IP-GZO and AP-CVD GZO.
C2-1-2 Materials Smart Design of Wide Bandgap ZnO: Function Core
Tetsuya Yamamoto, Hisao Makino, Huaping Song (Kochi University of Technology, Japan)
We propose a materials smart design of wide bandgap ZnO films with functional cores for their wide applications including optoelectronic devices such as flat display panels, thin film solar cells and short-wavelength light emitting diodes. Very recently, we reported a successful fabrication of 20-inch liquid crystal display (LCD) TVs mounted with indium-free Ga-doped-ZnO (GZO)-based common electrodes on RGBY(Red, Green, Blue, Yellow) color filters . The most important feature of the LCD TVs is high luminosity compared with conventional LCD TVs utilizing ITO (Indium Tin Oxide)-based common electrodes. In addition, we have established flexible plastic substrates with low-resistivity GZO films for flexible electronics substrates . In this work, we will discuss what a key ingredient in the recipe for success is. Nevertheless, the issue about the factors limiting carrier transport of polycrystalline n-type GZO films as well as Al-doped ZnO (AZO) is still open and is an object of theoretical and experimental investigations . A resolution to the issue can provide us with a novel science & technology to achieve not only highly durable ZnO-based transparent conductive films but also p-type ZnO films. We will propose the materials smart design in terms of a codoping method introducing functional core into host materials to realize moisture resistant n-type ZnO for use in gas sensors and to achieve p-type ZnO .
 N. Yamamoto, H. Makino, S. Osone, A. Ujihara, T. Ito, H. Hokari, T. Maruyama and T. Yamamoto, Thin Solid Films, 520 (2012) pp. 4131–4138.
 K. Nagamoto, K. Kato, S. Naganawa, T. Kondo, Y. Sato, H. Makino, N. Yamamoto and T. Yamamoto, Thin Solid Films, 520 (2011) pp. 1411-1415.
 T. Yamamoto, H. Song, H. Makino and N. Yamamoto, ECS Transactions, 45 (2012) pp.401-410.
 T. Yamamoto, physica status solidi (a) 193, No.3 (2002) 423-433; Thin Solid Films, 420-421 (2002) 100-106; Jpn. J. Appl. Phys. Vol. 42 (2003) pp. L514-L516.
C2-1-4 Electrical Properties of the ZnO Thin Films Grown on a-plane Sapphire Substrates using Catalytically Generated High-energy H2O
Naoya Yamaguchi, Tomohiko Takeuchi, Eichi Nagatomi, Takahiro Kato (Nagaoka University of Technology, Japan); Hironobu Umemoto (Shizuoka University, Japan); Kanji Yasui (Nagaoka University of Technology, Japan)
ZnO is highly useful for various applications such as short-wavelength optoelectronics and transparent conductive electrodes. Although CVD methods has many advantages for industrial applications, deposition of high-quality ZnO thin films by conventional CVD methods consume a lot of electric power for the reaction of source gases and for increasing the substrate temperature. In order to save energy and resources, a more efficient reaction of the oxygen and metalorganic source gases during film growth is highly desired. In a previous paper, we reported a new growth method for preparing ZnO films by reacting dimethylzinc and high-energy H2O generated from the Pt-catalyzed exothermic H2 and O2 reaction . It was also reported that ZnO films with excellent crystallinity and optical properties, as well as large electron mobility (mH) were grown on a-plane (11-20) sapphire (a-Al2O3) substrates. However, from the dependence of the electrical properties on the film thickness, these ZnO films are considered to consist of an interfacial layer with a high defect density (degenerate layer), generated due to the large lattice mismatch between ZnO and Al2O3 substrates, and an upper layer with a low defect density. In this paper, the electrical properties of the ZnO films are reported and analyzed according to a two-layer Hall-effect model and a two-donor model .
The ZnO epitaxial films were directly grown on a-Al2O3 substrates at temperatures of 773-873 K without any buffer layer. The film thickness was between 0.1-4.5 mm. The mH at room temperature (RT) increased from 30 to 190 cm2V-1s-1 with increasing film thickness to approximately 3 mm. From the temperature dependence of the mH, the mH increased significantly with decreasing temperature to approximately 100-150 K, but decreased at temperatures less than 100 K for films greater than 500 nm in thickness. The mH of the ZnO film (190 cm2V-1s-1) at RT increased to 660 cm2V-1s-1 at 100 K, but decreased at less than 100 K. In contrast, the mH hardly changed with temperature for films lesser than 500 nm in thickness. The mH and electron concentration of the upper layer were corrected based on the above results, assuming that the degenerate layer was 100 nm in thickness. Hydrogen and boron atoms were detected on the order of 1018 cm-3 and 1017 cm-3, respectively, by secondary ion mass spectroscopy. These atoms are considered to be donor impurities in ZnO. Therefore, temperature dependence of the electron concentrations of the ZnO films was analyzed using the two-donor model.
 K. Yasui et al., MRS Proc. (2011), Vol. 1315, pp. 21-26.
 D. C. Look et al., Phys. Rev. Lett., 82, No. 12 (1999) 2552.
C2-1-5 PEDOT:PSS Film having High Catalytic Activity for use as a Counter Electrode in Dye-sensitized Solar Cell
Chih-Ching Chang, Li-Chieh Chen, Dillip Mishra, Jyh-Ming Ting (National Cheng Kung University, Taiwan)
The replacement of Pt coated indium tin oxide (ITO) by poly-(3, 4-ethylenedioxythiophene):poly-(styrenesulfonic acid) (PEDOT: PSS) has been investigated. Commercial PEDOT:PSS was used as the starting material. Triton X-100 which was used as a surfactant were added to the PEDOT:PSS at different percentages, ranging from 0 to 10 wt%. The resulting films were characterized by Raman spectroscopy and UV-vis-NIR optical spectroscopy for the structure and optical absorption, respectively. Cyclic voltametry was also performed to determine the catalytic activity. The hole concentration and mobility were determined using Hall measurement. We show for the first time that the addition of Triton increases the catalytic activity, which was found to peak 5 wt%. The resulting films were also used as counter electrodes in rigid and flexible dye-sensitized solar cells (DSCs). The cells were evaluated using electrochemical impedance spectroscopy, solar simulator, and incident photon-to-electron conversion efficiency. The results also peak at 5 wt%. The Triton-added PEDOT:PSS counter electrode shows low series resistance and low charge-transfer resistance which approach that of plantium. We demonstrate that DSC with 5% Triton-added PEDOT:PSS counter electrode exhibits an efficiency of 4.62%, which is higher than that the DSC with Pt/ITO counter electrode.
C2-1-6 Formation and Characterization of CIS Thin Films by Co-sputtering Using CuSe2 and InSe2 Targets
Esso-ekazi Bleza, JiEun Jeon, Woo-Sun Lee, Nam-Hoon Kim (Chosun University, Korea)
Chalcopyrite CuInSe2 (CIS) thin films have been used as the absorber layer in the heterostructured thin film solar cell applications owing to their high absorption coefficient and the appropriate band gap energy of 1.04 eV. CIS thin films were generally prepared by using co-evaporation which should require the high-costly equipments for selenization process with hardness to control the accuracy in deposition rate for each element source. Non-selenization process was proposed and demonstrated for preparing CIS thin films without any Se- / S-containing gas in our previous study; the sputtering method was employed for the multilevel stack-structures by using In and CuSe2 alloy targets followed by rapid thermal annealing (RTA) in the N2 ambient. However, the chemical composition ratio of Se was lower than that of the required value for the high-efficiency photovoltaic devices. In this study, the co-sputtering method was used for the accurate control of chemical composition ratio by using CuSe2 and InSe2 selenide-targets as the starting materials to prepare the CIS thin films. The structural studies were examined by using some analytical techniques such as X-ray diffraction (XRD) and Raman scattering to confirm the CIS chalcopyrite phases in the RTA-treated thin films. UV-visible spectrophotometer and Hall Effect measurement system were employed to analyze the optical and electrical properties of CIS thin films fabricated by this non-selenization process with the co-sputtering method. The change of chemical composition in the RTA-treated CIS thin films was analyzed by using the secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) with a change of the sputtering power to InSe2 target to verify the relationship between sputtering power to InSe2 target and stoichiometry of CIS thin films by analyzing the chemical composition ratios of In / Cu and Se / Cu. The optimum sputtering power to InSe2 target was obtained by considering the structural, chemical, optical, and electrical properties of CIS thin films.
C2-1-7 Optical Properties of Sputter-Deposited Germanium Oxide (GeO2) Films
Chintalapalle Ramana (University of Texas at El Paso, US); Neil Murphy, Lirong Sun, John Jones, Rachel Jakubiak (Air Force Research Laboratory, Materials and Manufacturing Directorate, US)
Germanium oxide (GeO2) exhibits many interesting physical, chemical and electronic properties for applications in a wide range of optical, electronic and optoelectronic devices. GeO2 is a photoluminescence and dielectric material. It exhibits high values of dielectric constant, refractive index, thermal stability and mechanical strength. Due to these fascinating optical and electronic properties, GeO2 has been considered as a promising material for optical waveguides and nano-connections in optoelectronic communications. Synthesis and optimization of a particular phase and compositional stability of GeO2 is very important as this material exhibits several polymorphs. Additionally, the optical, photochemical and optoelectronic properties of metal oxide films are sensitive to the processing conditions such as base pressure, growth temperature, reactive pressure (if any), deposition rate and annealing conditions. Therefore, the controlled growth and manipulation of specific crystal structures of GeO2 at the nanoscale dimensions has important technological implications. In the present work, GeOx films were grown by the direct-current (DC) magnetron sputter-deposition employing Ge target for reactive deposition. The effect of oxygen gas flow rate on the structure and optical properties has been investigated. The deposition was made under the reactive atmosphere of argon (Ar) and oxygen (O2) at a constant pressure of 5 mTorr. The Ar and O2 were controlled using as MKS mass flow meters. While the total gas flow rate is 20 sccm, O2 flow rate is varied from 0 to 20 sccm balance Ar. The deposition was carried out to obtain a ~100 nm thick GeOx film. The grazing incidence X-ray diffraction and scanning electron microscopy analyses confirm that the GeOx films grown were amorphous. The chemistry of Ge ions exhibit an evolution from pure Ge films to Ge+GeO2 mixed phase and then finally to GeO2 composition with increasing oxygen gas flow rate from 0 to 20 sccm. The optical properties primarily probed by the spectroscopic ellipsometry indicate that the effect of oxygen gas flow rate is significant on the optical constants of GeOx films. The measured index of refraction (n) at λ=550 nm is 4.67 for films grown without any oxygen indicating the characteristic behavior of Ge semiconductor films. For O2 flow rate of 5 sccm, mixed Ge+GeO2 films exhibit a decrease in n value to 2.62. Finally n drops to 1.60 for oxygen flow rates of 10-20 sccm, where the films characterizing by the fully oxidized state of Ge. The results and detailed analysis will be presented.
C2-1-8 Experimental and Theoretical Analysis of Solar Absorbing Mo-SiO2 Cermet Coating
Zhuopeng Tan, Ji Zhou (Tsinghua University, China); Dongmei He, Fuyun Zhou, Jingqing Yi (Camda Institute of New Energy Technology, China)
Solar spectrally selective absorbing Mo-SiO2 metal-dielectric (cermet) coatings with a four layer structure were fabricated by using a sputtering technique. It consists two Mo-SiO2 cermet layers on top of a pure Mo layer and one SiO2 layer on top of the cermet layers. A method combined theoretical simulations and experiments were proved to be efficient and effective on identifying candidate film structures. Effects of film thickness, compositions and substrate surface roughness on film optical properties were studied. Both cermet layer thickness and compositions have large effects on the film absorptance and emission over the other factors. For the substrate surface roughness, it was found that the smoother it was, the less emittance the film would have. Method of thermal shock for temperatures ranging from 473 K to 773 K was employed to test the thermal stability under pressure of 0.1 Pa of the selected structure. Based on the experimental results, the selected cermet coating has solar absorptance over 0.94 and keeps stable after thermal shock for over one thousand cycles.
C2-1-9 Effect of Nitrogen Incorporation on the Optical, Structural and Electrical Properties of Indium Zinc Oxide.
Jose Ortega (Universidad Autónoma de San Luis Potosí, Mexico); Miguel Aguilar-Frutis (Instituto Politécnico Nacional, Mexico); Ciro Falcony (Instituto Politecnico Nacional, Mexico); Victor Méndez-García (Universidad Autónoma de San Luis Potosí, Mexico); Jesús Araiza (Universidad Autónoma de Zacatecas, Mexico)
Transparent IZO thin films were deposited on Si(100) substrates by RF reactive magnetron sputtering at room temperature under different nitrogen concentration in the plasma and reactive atmosphere. As precursor, an IZO target (In2O3-ZnO, 90-10 wt%) with a purity of 99.99% was used. The effect of the incorporation of nitrogen on the structural, electrical and optical properties was amply studied. Energy Dispersive Spectroscopy (EDS) confirms the presence of In, Zn, O and N in all the deposited films. The IZO:N films maintained the amorphous structure even after a gas flow ratio of 15 sccm N2/5 sccm Ar. The electrical resistivity, mobility and carrier concentrations were determined from Hall Effect measurements using the Van der Pauw configuration. The lowest resistivity obtained was 3.8x10-4 Ω·cm with a mobility of 31.9 cm2/V·s, and carrier concentration of 5.1x1020cm-3. By measurement of absorption and transmission Uv-vis spectroscopy in conjunction with the characterization by spectral ellipsometry (SE), the optical properties were analyzed according to the deposition conditions. For SE analysis were used two dispersion models, the Classical and Adachi models. The optical parameters obtained using both dispersion models presents a notable increase in the optical parameters, refractive index and extinction coefficient, related to the increment of nitrogen in the film. The band gap (Eg) of the films depends strongly on the nitrogen incorporation and as a direct result, Eg presents a narrowing from 3.5 to 2.5 eV, associated with the Burstein-Moss effect as well as band gap renormalization due to the interaction electron-electron and electron-impurity.