ICMCTF2013 Session C2-1: Fundamentals of Thin Films towards Optoelectronics Devices

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 (T_S) of 200 ºC. GZO films with the thicknesses of 1-5 μm were prepared on r-plane sapphire substrates at T_S =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.

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 [1]. 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 [2]. 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 [3]. 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 [4].

[1] 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.

[2] 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.

[3] T. Yamamoto, H. Song, H. Makino and N. Yamamoto, ECS Transactions, 45 (2012) pp.401-410.

[4] 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.

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 [1]. It was also reported that ZnO films with excellent crystallinity and optical properties, as well as large electron mobility (m_H) 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 [2].

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 m_H 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 m_H, the m_H 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 m_H 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 m_H hardly changed with temperature for films lesser than 500 nm in thickness. The m_H 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.

[1] K. Yasui et al., MRS Proc. (2011), Vol. 1315, pp. 21-26.

[2] D. C. Look et al., Phys. Rev. Lett., 82, No. 12 (1999) 2552.

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.

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, GeO_x 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 GeO_x film. The grazing incidence X-ray diffraction and scanning electron microscopy analyses confirm that the GeO_x 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 GeO_x 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.

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 (E_g) of the films depends strongly on the nitrogen incorporation and as a direct result, E_g 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.