ICMCTF2011 Session C2/F4-2: Thin Films for Photovoltaics and Active Devices: Synthesis and Characterization

Tuesday, May 3, 2011 8:00 AM in Room Sunset

Tuesday Morning

Time Period TuM Sessions | Abstract Timeline | Topic C Sessions | Time Periods | Topics | ICMCTF2011 Schedule

Start Invited? Item
8:00 AM C2/F4-2-1 Influence of Sputtering Powers on the Characteristics of ZnO:B Thin Films
Lian-Hong Wong, Yi-Sheng Lai (National United University, Taiwan); D.S. Wuu (National Chung Hsing University, Taiwan); Jyh-Liang Wang (Ming Chi University of Technology, Taiwan)
ZnO:B thin films are deposited using ZnO-B2O3 (3 wt%) sputtering targets at rf powers ranging from 80 W to 180 W. It is found that the deposition rate and crystallinity increase with increasing rf powers. From θ-2θ XRD, it is found that the (002) diffraction peak shifts to low 2θ angle as the rf power increases. The transmittances of ZnO:B thin films are around 90% in the visible light region. The optical bandgap of ZnO:B thin film increases as the rf power increases. The ZnO:B films deposited at rf power of 180 W exhibit good electrical properties with resistivity of 4.46×10-3 Ωcm, carrier concentration of 1.15× 1021 cm-3, and carrier mobility of 15.03 cm2/Vs, respectively.
8:20 AM C2/F4-2-2 High Mobility Transparent Conducting Oxides: A Modulation Doping Approach
Sunnie Lim, Rueben Mendelsberg, Andre Anders, KinMan Yu (Lawrence Berkeley National Laboratory)

Transparent conducting oxides (TCOs) are playing an important role in today's opto-electronic devices. However in recent years, there has been an increasing demand for high mobility TCO to satisfy the need of next generation technologies such as solar cells and low emissivity windows coating. The carrier mobilities of conventional single material metal oxides are physically limited by various scattering mechanisms. While most of these scattering events can be overcome by careful tailoring of the microstructure, the main limitation in the mobility of TCO is due to ionized impurity scattering. This scattering arises when charge carriers are deflected by the electrostatic field associated with the dopants.

In this presentation, we will describe the major scattering mechanisms governing the electron transport relevant to TCOs. We will introduce a technique of modulation doping which involves a multilayer thin film system which can overcome the limitations of ionized impurities scattering. We will present experimental results from electrical characterization and microstructure studies from TEM of modulation doping based on ZnO deposited by pulsed filtered cathodic arc deposition.
8:40 AM C2/F4-2-3 Dominant Factors Determining Moisture Resistance of Highly Transparent Conductive Ga-Doped ZnO Films
Yasushi Sato, Takahiro Yamada, Hisao Makino, Naoki Yamamoto, Tetsuya Yamamoto (Kochi University of Technology, Japan)

Ga-doped ZnO (GZO) films have attracted attention for use in window layers of solar cells and transparent electrodes of liquid crystal display panels. We investigated the dominant factors determining moisture resistance of GZO films. GZO films with different Ga contents (1.5, 3.0 and 4.0 wt.%) were deposited on glass substrates at 200 °C by ion-plating deposition using dc-arc discharge.1-3 To control the factors determining resistivity of the films; carrier density and Hall mobility, oxygen gas flow rate (fO2) was changed in the range of 6 to 30 SCCM. The resistivity of 3.0 wt.% GZO films with a carrier density of 5.8 × 1020 cm-3 and Hall mobility of 31 cm2V-1s-1 was 3.4 × 10-4 Ωcm and changed by less than 3% over a 500 h reliability test at a temperature of 60 °C and a relative humidity of 95%. Optical transmittance in the wave length from 450 to 1200 nm of the sample was above 80% . The resistivity of all the samples changed by less than 25% confirmed by the reliability test described above. On the basis of the analysis of the data obtained by Hall effect measurements, we concluded that the dominant factors determining moisture resistance of GZO films is carrier density, The maximum carrier density, nmax, needed for moisture resistant GZO films within the change of less than 10% is 7 × 1020 cm-3. The resistivity of GZO films with a carrier density of 1.1 × 1021 cm-3 larger than nmax changes by about 20%. We will further discuss the difference in the contribution to the moisture resistant properties between extrinsic defects, Ga at Zn site, and intrinsic defects including Zn interstitial and oxygen vacancy.

This work was supported by New Energy and Industrial Technology Development Organization (NEDO) under the National Project of Rare Metal Indium In Substitute Materials Development.

1 T. Yamada et al., Appl. Phys. Lett. 91, 051915 (2007)

2 T. Yamada et al., J. Appl. Phys. 107, 123534 (2010)

3 N. Yamamoto et al., J. Electrochem. Soc. 157, J13 (2010)

9:00 AM C2/F4-2-4 Changes in Electrical and Optical Properties of Polycrystalline Ga-Doped ZnO Thin Films Due to Thermal Desorption of Zinc
Hisao Makino, Yasushi Sato, Naoki Yamamoto, Tetsuya Yamamoto (Kochi University of Technology, Japan)
Ga-doped ZnO (GZO) film is one of promising candidate as ITO substitute materials for transparent electrodes in optoelectronic applications, such as, flat panel displays and solar cells. In this paper, we studied changes in electrical and optical properties of transparent conductive GZO films due to thermal desorption of zinc. 150 nm thick GZO films were deposited on glass substrates by an ion-plating with DC-arc discharge at substrate temperature of 120°C. Resistivity of the as-deposited GZO film was 2.5 x10-4 Wcm with carrier concentration of 1.2 x 1021 cm-3 and Hall mobility of 20 cm2/Vs. Desorption of Zn from the GZO films was characterized by thermal desorption analysis (TDA). Amounts of desorbed Zn were evaluated by integral intensity of the TDA signals. In order to control amounts of desorbed Zn, the TDA measurements were interrupted at several temperatures. Although the resistivity showed little change after the TDA measurements, carrier concentration decreased and Hall mobility increased with increasing the interrupted temperature. It was found that the change in carrier concentration showed good correlation with the Zn desorption. The decrease in carrier concentration linearly increased with increasing the amount of desorbed Zn. It is suggested that the decrease in carrier concentration is due to desorption of interstitial zinc acting as native donors in ZnO. Optical absorption spectra showed a fundamental optical gap in ultra violet region and a free-carrier absorption in the mid-infrared region. In addition, we observed an optical absorption band in visible wavelength region due to some defects. After the TDA measurements, the intensity of optical absorption band linearly decreased with increasing the amount of desorbed Zn. It was suggested that the optical absorption band is most probably due to some Zn-related defects in the GZO films.
9:20 AM C2/F4-2-6 Bending Properties of Transparent Conductive Ga-doped ZnO Films
Koichi Nagamoto, Kunihisa Kato, Satoshi Naganawa, Takeshi Kondo (LINTEC Corporation, Japan); Yasushi Sato, Hisao Makino, Naoki Yamamoto, Tetsuya Yamamoto (Kochi University of Technology, Japan)

Transparent conductive oxides (TCO) on polymer substrates are prospected as a key material for next-generation devices such as flexible displays and photovoltaics. The advantages of polymer substrates include light-weight, low cost, a multiplicity of materials with tailored properties, shock absorption and highly flexibility. However, polymer substrates also have disadvantages such as low heat resistance and large thermal expansion coefficient compared with glass substrates. A main challenge for an efficient TCO on polymer substrate is not only to choose conductive oxide materials having capability of growing at low substrate temperature, but also to develop a deposition processes in order to obtain good electrical characteristics. Thus, in this study, we developed a multiple-deposition method to achieve deposit TCO on polymer substrates with lower than 100°C. The influences of process temperature on structural and electrical characteristics of 100 nm thickness Ga-doped ZnO (GZO) film on polymer substrates were investigated. In addition, the effects of bending strains on the characteristics were also discussed.

The structural, electrical, optical and bending properties of highly transparent conductive polycrystalline GZO films deposited on polyester substrates at various temperatures of less than 100 °C by ion plating with dc-arc discharge were investigated. We developed a multiple-deposition method to achieve controllable different polyester substrate temperatures. Analysis of data obtained from XRD measurements and cross-sectional SEM images show that polycrystalline GZO films with wurtzite structure exhibit highly (0002) orientation perpendicular to the substrate. The resistivity and average transmittance in the visible wavelength region of GZO films on polyester substrates were 5.0 x 10-4 ohm-cm and more than 80 %, respectively.

The mechanical bending properties of the GZO films were investigated on the basis of analysis of data obtained by comparing sheet resistance between before and after a bending test. The bending test was carried out for the GZO films deposited at various process temperatures. The GZO films deposited at lower substrate temperatures exhibit improved bending performance.

We have demonstrated that our multiple-deposition method to achieve controllable different polyester substrate temperatures works very well to control the bending properties of GZO films.

The financial support from the Japan Science and Technology Agency is gratefully acknowledged

9:40 AM C2/F4-2-7 H-Bonded Effects and Properties of Novel Supramolecular Nanocomposites Containing Aryl-Imidazo-Phenanthroline -Based Metallo- Polymer H-Donors and Surface-Modified ZnO Nanoparticle H-Acceptors
Hsiao-Ping Fang, Hong-Cheu Lin (National Chiao Tung University, Taiwan)

Abstract:

Four novel metallo-polymers (P1-P4 as proton donors) were synthesized to study for the H-bonded effects on their nanocomposites to blend with pyridyl ZnO nanoparticles (ZnOpy as proton acceptors). In order to investigate the nanocomposites containing metallo-polymers P1-P4 and surface-modified ZnOpy nanoparticles, nanocomposites P3/ZnOpy and P4/ZnOpy were characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and Transmission electron microscopy (TEM) analyses. In the XRD measurements, the introduction of surface-modified ZnOpy nanoparticles to metallo-polymers P3-P4 has induced higher crystallinities in their nanocomposites P3/ZnOpy and P4/ZnOpy. In the TEM studies, due to H-bonds formed between metallo-polymer P3 and ZnOpy nanoparticles, ZnOpy nanoparticles were more homogeneously dispersed in metallo-polymer P3, which was not well dispersed in nanocomposite P1/ZnOpy (without H-bonds). Finally, it suggested that these nanocomposites might become useful materials for future photovoltaic applications.

10:00 AM C2/F4-2-8 Non-vacuum Process of ZnO Thin Films Grown by Spray Pyrolysis Technique
Kenji Yoshino (University of Miyazaki, Japan); Yujin Takemoto, Masatoshi Shinmiya, Minoru Oshima (University fof Miyazaki, Japan); Kouji Toyoda, Koichiro Inaba, Kenichi Haga, Koichi Tokudome (Tosoh-finechem Co. Ltd., Japan)

Transparent conductive oxide (TCO) materials have attracted much attention for use in liquid crystal displays and photovoltaic devices. In particular, Sn-doped In2O3 (ITO) is known as a good transparent conductive oxide material. Recently, ZnO has also been studied as a TCO material because material (Zn) cost is very low in comparison to that of ITO (In). ZnO has shown promise for many applications including gas sensors, transport electrodes, piezoelectric devices, varistors and surface acoustic wave devices. Its direct optical band gap of 3.4 eV at room temperature is wide enough to transmit most of the useful solar radiation in ZnO/CuInSe2 based solar cells. Furthermore, ZnO is a good candidate to substitute for ITO (In-doped In2O3) and FTO (F-doped SnO2) in transparent conductive electrodes. Many techniques have been employed to produce the ZnO thin films including molecular beam epitaxy, metal organic chemical vapour deposition, radio frequency magnetron sputtering, spray pyrolysis and sol-gel methods.

Non-vacuum processes such as spray and sol-gel method are effective for thin film growth because damage to the surface due to plasma is avoided, high vacuum is not required, and equipment costs are low. Furthermore, low temperature growth of ZnO is important for compatibility with photovoltaic device fabrication processes.

In this work, undoped ZnO films on glass substrates were grown by a spray pyrolysis method at room temperature (RT, ~ 300˚C). Polycrystalline ZnO thin films were successfully grown at RT under an air atmosphere. Diethylzinc (DEZ) was used as the Zn source material. The DEZ solution was diluted by some solvent in order to use safely under an air atmosphere. X-ray diffraction indicates that (10-10) and (10-11) peaks are dominant. The lattice constants of the a and c axes are larger than that of ICDD data. The samples develop a c axis (0002) orientation with increasing substrate temperature. Furthermore, the lattice constants of the a and c axes become closer to those of ICDD data with increasing substrate temperatures.

10:20 AM C2/F4-2-9 Electrical Conductivity Enhancement of Nb-Doped TiO2 Sputtered Thin Films by a Post Hot-Wire Annealing in a H2 Atmosphere
Carlos Tavares, Maria Castro, Pedro Alpuim, Emilio Marins, Anura Samantilleke, Stanislav Ferdov, Maria Benelmekki (University of Minho, Portugal); E. Xuriguera (Universitat de Barcelona, Spain)
In this work Nb-doped TiO2 thin films were deposited by dc-pulsed reactive magnetron sputtering from a composite Ti:Nb target, using oxygen as reactive gas. In order to optimise the electrical conductivity of the as-deposited thin films, these were subsequently heat-treated by atomic hydrogen at 500ºC. The hydrogen flow was generated by a hot-wire filament, inside a high-vacuum CVD reactor, at a temperature of 1750ºC. The hydrogen pressure was varied between 10 and 500 mTorr, for different annealing times. A 2-orders of magnitude increase in conductivity was typically observed for optimised hydrogen treatments. Dark conductivity (σdk) and its activation energy were measured as a function of (inverse) temperature and the value of σdk at room temperature was used to assess the effect of the H2 annealing on the transport properties. Carrier mobility and resistivity were also investigated using Hall effect measurements. Correlations between structural and electrical properties and the hydrogen treatment conditions are discussed. The purpose of these coatings is to provide a transparent and conductive front contact layer for a-Si based photovoltaics, with a refractive index that better matches that of single and tandem solar cell structures.
10:40 AM C2/F4-2-10 Structural and Electrical Properties of Sol-Gel Derived Yttrium Oxide Dielectric Films
Chien-Yie Tsay, Chia-Hsiang Cheng (Feng Chia University, Taiwan); Yu-Wu Wang (National Changhua University of Education, Taiwan); Chi-Jung Chang, Chung-Kwei Lin (Feng Chia University, Taiwan)

Metal oxide films are utilized as the gate dielectrics for ZnO-based thin film transistors (TFTs) can improve the characteristics of those devices. In this study, transparent dielectric films of yttrium oxide (Y2O3) were deposited onto alkali-free glass substrates (NEG OA-10) by a sol-gel spin coating process . The authors report on the effect of annealing temperature on the microstructural, optical and electrical properties of sol-gel derived Y2O3 dielectric films. Each as-coated film was preheated at 300oC for 10 min, and then annealed at a temperature rage from 400 to 550oC for 1 h under air ambiance. XRD results revealed that annealed Y2O3 films were polycrystalline with a cubic structure, exhibited preferential orientation along the (222) plane, and the crystallinity level of Y2O3 films enhanced with annealing temperature increased. Those as-coated films annealed at temperature above 450oC had high optical transparency with average transmittance over 88.0% in the wavelength region of 400-1000 nm. Moreover, chemical compositions of sol-gel derived Y2O3 dielectric films were examined. The metal-insulator-metal (MIM) capacitor structure, Al/Y2O3/ITO/glass, were fabricated for examining film’s electrical properties, namely dielectric constant, leakage current, and breakdown field, that were evaluated by capacitance-voltage (C-V) and current-voltage (I-V) measurements.

11:00 AM C2/F4-2-11 Preparation, Synthesis Techniques and Some Properties on CdMnS Diluted Magnetic Semiconductor Thin Films
Jaiprakash Dargad (Dayanand Science College, India)
Highly oriented Cd1-xMnxS thin films were deposited onto the glass substrates by a chemical growth process. During deposition concentration of Mn2+ (0 ≤ x ≤ 0.5) was varied between 0 to 0.5. Layer thickness measurement, electrical and optical properties of these films were examined. Absorption spectra were used to investigate the optical band gap of Cd1-xMnxS films. An increase in the band gap of CdMnS films from that of the bulk CdS with increasing concentration of Mn2+ has been observed (2.34eV to 2.8eV). The growth mechanism and reaction kinetics have been suggested and discussed for these films. Chemical composition of the films was analysed by an EDS technique. The analysis showed that Mn2+ replaces Cd2+ from the lattice. No considerable removal of S2- from the CdS lattice has been observed. The surface morphology studies showed polycrystalline nature of the samples and for lower concentration of Mn2+ in CdS (up to 0.01 mol %), the films become bit coarser whereas then become diffused and less crystalline at higher concentration of Mn2+ in CdS. The room temperature electrical conductivity decreased with an increase in Mn2+ concentration in CdS. The XRD studies showed structure of Cd1-xMnxS thin film exibits hexagonal wrutzit structure. A phtoelectrochemical cell formed with an active Cd1-xMnxS semiconductor photoelectrode and charectarised to determine various cell parameters.
11:20 AM C2/F4-2-12 Structural, Electrical and Optical Properties of AgInTe2 Films Grown by a Hot Wall Technique
Amarjit Singh, R.K. Bedi (Guru Nanak dev University, Amritsar, India)
Silver indium telluride (AgInTe2) films have been deposited by hot wall technique onto a glass substrate kept at different temperatures in a vacuum of 10-5 torr. Experimental conditions were optimized to obtain better crystallinity in the films. The films so prepared were studied for their structural, electrical and optical properties. X-ray diffraction (XRD) pattern indicate that the prepared films are polycrystalline and are highly oriented in the (111) direction. Scanning electron micrographs (SEM) of the films show an increase in crystallite size with increasing substrate temperature. Observations reveal that the electrical conductivity of films increases with increase in substrate temperature. The band gap of the films was found to be lie in range 1.11- 1.14 eV and 1.64- 1.68 eV, which is due to the fundamental absorption edge and transition originating from crystal field splitting respectively.
Time Period TuM Sessions | Abstract Timeline | Topic C Sessions | Time Periods | Topics | ICMCTF2011 Schedule