ICMCTF2013 Session C4-1: Thin Film Growth and Characterization for Optoelectronic Devices

Wednesday, May 1, 2013 8:00 AM in Room Sunset

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8:00 AM C4-1-1 Fabrication and Characterizations of CIGS Films Using One-step Electrochemical Co-deposition Methods
Yih-Min Yeh (WuFeng University, Taiwan); Hsiang Chen (National Chi-Nan University, Taiwan, Republic of China); Song-Min Liu (WuFeng University, Taiwan); ShengTing Huang, YuJie Chen (National Chi-Nan University, Taiwan, Republic of China)

Due to the advantages of low cost, fast and large-area production, electrodeposition has become one of the mainstream fabrication methods of the CIGS solar cells. In this study, CIGS film was formed using one-step electrochemical deposition at various deposition potential voltages in solution with diverse pH values. M ultiple material analyses such as X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) images, energy diffraction spectrometer (EDX), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) were used to examine the electro-deposition parameters. The results reveal that the Ga could not be deposited on the substrate in stirred conditions. In unstirred conditions, quaternary CuInGaSe compound could be formed with various Cu/In/Ga/Se ratios grown in various deposition parameters. The experimental results indicate that the pH value of the solution and electric potential play important roles on determining the composition of the CIGS film. UV spectrometer was performed to measure the bandgap of the CIGS thin film. The electrodeposited CIGS film shows promises for future solar cell applications.

8:20 AM C4-1-2 Preparation of CdMnS Thin Film: Applications in Photoelectrochemical Cell
Jaiprakash Dargad (Dayanand Science College, Latur, Swami Ramanand Teerth Marathwada University, Nanded, Maharashtra, India)

A new class of dilute magnetic semiconductor (DMS, CdMnS) thin film based photoelectrochemical cell is presented. DMS thin films are synthesized on both glass and stainless steel substrates using a chemical growth process. The preparation parameters (such as growth temperature, time, reaction pH, precursor concentrations, etc) were optimized to yield characteristically oriented films. The layer thickness was found to be decreased with an increase in Mn2+ concentration. The composition of the as-grown samples was determined by an EDS technique. The electrochemical cells were then formed out of these series of films as the active photoelectrodes, an electrolyte and a counter electrode. The cells were then characterized through their dark and photosensing properties. The other cell parameters were determined from these studies and the cell performance has been evaluated with a special reference to Mn2+ concentration in CdS. A significant enhancement in performance has been observed for a cell with electrode composition of x = 0.01.

8:40 AM C4-1-3 Fabrication and Characterization of High-efficiency CdTe-based Thin-Film Solar Cells
Yanfa Yan, Naba Paudel (The University of Toledo, US)
CdTe-based thin-film solar cells with conversion efficiencies better that 15% have been fabricated on commercial Pilkington SnO2:F-coated soda-lime glass without anti-reflection coating. The CdS window layers were grown by either magnetron sputtering or chemical bath deposition. The CdTe absorber layers were synthesized by close-spaced sublimation method. Cu/Au layers were used as the back contacts. We have achieved cells with open circuit voltage higher than 0.840 V and fill factor better that 75%. The structure of the cells and the quality of the CdTe thin films have been characterized by electron microscopy, X-ray diffraction, and optical spectroscopy. The correlation between the cell performance and material quality will be discussed in the presentation.
9:20 AM C4-1-5 The Optimization of Indium Codoping Concentration in 100-nm-thick GZO Films for Low Resistivity and High Humidity Resistance Properties
Huaping Song, Hisao Makino, Naoki Yamamoto (Kochi University of Technology, Japan); Seiichi Kishimoto (Kochi National College of Technology, Japan); Tetsuya Yamamoto (Kochi University of Technology, Japan)
Ga-doped ZnO (GZO) films are promising transparent conductive oxide (TCO) films for use in electrodes of flat display panels and window layers of thin film solar cells. Control of the humidity resistant properties of GZO films is a crucial research topic for practical wide applications, including optoelectronic devices with substrates based on flexible polymer materials having highly water-absorption coefficients in near future. For 200-nm-thick GZO films deposited on glass substrates, we have optimized oxygen gas flow rates (OFRs) introduced in the chamber during the deposition, and achieved humidity resistant GZO films; The relative change in electrical resistivity ρ after 500 hours humidity test (temperature of 60 °C and relative humidity of 95%) could be controlled to be less than 10%. Nevertheless, for wide applications, an issue about how to obtain durable thinner GZO films is still open. For as-deposited 100-nm-thick GZO films with a ρ of 3.71 μΩm, the relative change in resistivity Δρ after the test was 32.6%. Very recently, we proposed a novel technology to co-doped indium (In) species together with the control of OFR to improve the humidity resistant properties of 100-nm-thick GZO films. As a result, for the 100-nm-thick GZO:In film with the ρ of 4.09 μΩm before the test, we have found a reduced Δρ of 12.3% after the humidity test. In this study, we optimized both the contents of In codoped and the magnitude of OFRs to achieve humidity resistant GZO films. GZO films with different contents of In codoped were grown on glass substrates at 200 °C by ion plating with dc arc-discharge under various OFRs ranging from 0 to 25 sccm. The In contents in the deposition source (the sintered ZnO tablets with a Ga2O3 content of 3 wt.%) were 0.25, 0.5 and 0.75 wt.%.Analysis of X-ray diffraction measurements results shows that for 0.25 wt.% In-codoped GZO films, an increase in OFRs gives rise to the deterioration of the alignment between c-axis orientation of columnar in polycrystalline GZO films. On the other hand, for 0.50 and 0.75 wt.% In-codoped GZO films, further increase in OFR of more than 15 sccm improves the alignment above accompanied with an increase in ρ. Analysis of data obtained by Hall effect measurements shows that an increase in In contents enhances the humidity resistant properties for GZO films at any given OFR. We optimized both In contents and OFRs not only to enhance the reduction of Δρ but also minimize ρ. As a result, we have ensured the durability of 100-nm-thick GZO:In films to humidity : Δρ=6.5% for GZO:In transparent films with low ρ of 4.54 μΩm after the humidity test.
9:40 AM C4-1-6 Effects of Native Defects on the Electrical and Optical Properties of Cadmium Oxide
KinMan Yu (Lawrence Berkeley National Laboratory, US); Lothar Reichertz (RoseStreet Laboratories, US); Sylwia Grankowska (Warsaw University, Poland); Douglas Detert, Oscar Dubon (University of California, Berkeley; Lawrence Berkeley National Laboratory, USA); André Anders, Wladek Walukiewicz (Lawrence Berkeley National Laboratory, US)

We have recently demonstrated that intentional doping of CdO results in an ideal uncompensated material with extremely low resistivity (ρ<10-4Ω-cm) and an excellent transmission window in the range from 400 nm to >1500 nm, making this material an ideal transparent conductor for photovoltaics with low band gap absorbers [1]. These exceptional electrical and optical properties were obtained typically in polycrystalline CdO synthesized by a variety of methods including pulsed laser deposition (PLD), filtered cathodic arc deposition and RF sputtering. Here, we present a systematic study of the electrical and optical properties of CdO thin films grown by RF sputtering. In particular we studied the effects of defects in undoped and In-doped CdO samples under different deposition and annealing conditions. We found that at low growth temperatures (<200oC), a fast growth rate tends to trap both oxygen vacancies and compensating defects in the film resulting in materials with high electron concentration of >2x1020/cm3 and relatively low mobility (~30-50 cm2/V-s). Reducing the growth rate to ~2-3 nm/min results in high quality material with electron concentration of ~1x1020/cm3 and mobility >100 cm2/Vs. In order to identify the dominating defects in the films, we have also carried out thermal annealing in N2 or O2 ambient. Annealing in O2 ambient consistently reduces electron concentration and increases mobility, suggesting that the dominating defects in sputtered CdO films are oxygen vacancies. Sputtering with an Ar/O2 (80%/20%) plasma results in high resistivity CdO films ( ρ>10-2Ω-cm) with low electron concentration of 1018/cm3, most likely due to the increased incorporation of excess O resulting in anon-stoichiometric material. However such material is not stable and an increase in the electron concentration is observed at brief annealing as short as 1 second at 300oC, suggesting outdiffusion of O and increase of the O vacancy concentration. This conclusion is further supported by more pronounced annealing effects observed in thinner samples. Intentional doping with In donors leads to an increase of both the electron concentration and the mobility. With proper doping CdO films with electron concentration of more than 1021 cm-3 and electron mobility higher than 100 cm2/Vs can be achieved, indicating greatly reduced ionized impurity scattering effects in in this material. We will also discuss the effect of defects and intentional doping on optical properties of as grown and annealed CdO films.

[1] K.M Yu, et al., J. Appl. Phys. 111 123505 (2012).

10:00 AM C4-1-7 Study of the Instability of Amorphous InGaZnO Thin Film Transistor under the DC and AC Drain-bias Stress
Li-Wei Lin, Ting-Chang Chang, Sheng-Yao Huang, Man-Chun Yang (National Sun Yat-Sen University, Taiwan, Republic of China); Kai-Hsiang Yang (University of Toronto, Canada); Ming-Hsin Wu, Min-Chen Chen, Ke Mai, Yi-Jen Chiu (National Sun Yat-Sen University, Taiwan, Republic of China); Bo-Liang Yeh (Advanced Display Technology Research Center, AU Optronics, Taiwan)
This study investigates degradation behavior of AC drain-bias stress under illumination for InGaZnO thin film transistors. In the previous study, we reported the gate-to-drain capacitance-voltage curve exhibited degradation of the hump phenomenon during the DC drain-bias stress. However, it observes different degradation behavior after the AC drain-bias stress, which the hump phenomenon disappeared but the Vth shift was observed. Significantly, the degradation behavior during the AC drain-bias stress is related with the duty-ratio and frequency of the AC pulse waveform. The experiment results indicate the pulse-width (PW) time during the drain bias makes the holes trapping at interface defect between insulator/active layer, and on the other hand during the puls-base (PB) time the hole trapping induces surface band banding make electron inject into the interface defect, resulting in the electron and hole recombine within the interface defect. Moreover, we calculate the charge trapping density and physics-position of charge trapping by using the capacitance-voltage curves. Finally, this work also employs different intensity of illumination during the stresses and the simulation system to further clarify the mechanism of degradation behavior.
10:20 AM C4-1-8 Microwave-assisted Hydrothermal Synthesized Nitrogen-doped TiO2 Photocatalysts for Enhanced Visible Light Response
Wei Chung Huang, Jyh-Ming Ting (National Cheng Kung University, Taiwan)

This work reports the synthesis and characterization of N-doped TiO2 mesoporous beads prepared by a two-cycle rapid microwave-assisted hydrothermal method using three different types of nitrogen dopants: diaminohezane, triethylamine, urea. In the first cycle, TiO2 mesoporous beads with controlled structures were synthesized at 200°C. The obtained beads were then subjected to a second cycle of microwave-assistaed hydrothermal process for dpoing with one of the aforementioned dopants. The sue of a secon cycle is to maintain the integrity of the beads, which otherwise would be easily destroyed if the synthesis and doping processes are carried out at the same time. The crystalline structure of the N-doped TiO2 was examinedusing X-ray diffraction. The surface state and structure were investigated using X-ray photoelectron spectroscopy and scanning electron microscopy, respectively. The absorption of N-doped TiO2 in the range of visible light was confirmed using UV-Vis spectroscopy. The self-assembled N-doped TiO2 redshift in adsorption edge up to 500nm. The obatined TiO2 was also dissloved in methyl blue solution to function photocatalyst and the catalytic activity was determined. The photocatalytic activity of all N-doped TiO2 can be found the N-doped TiO2 used dianimohexane as the nitrogen dopant decompose the organic pollution more complete and rapid than others .

10:40 AM C4-1-9 Effect of Thermal Annealing on Nickel Oxide Doped AZO Transparent Conducting Thin Films Prepared by DC Magnetron Sputtering System
Young-Dae Jo (Pusan National University, Republic of Korea)
The NiO doped AZO (NiO:AZO) thin films with different NiO weight percent range between 0 and 5.0 wt. % were synthesized by DC magnetron sputtering method using composite targets in argon atmosphere at room temperature, and then forming gas annealing process was carried out at 400°C for 1hour. The electrical, structural and optical properties were detailedly and extensively studied. The electrical resistivity of as-deposited NiO:AZO films was found to increase with increasing of NiO concentration due to the effect of decreasing of electrical mobility was more considerable than enhancing of carrier concentration. After N2/H2 annealing process, the electrical resistivity was remarkably reduced to 7.67× 10-4 Ωcm which is the best value of this study at 3.0 wt. % NiO doping concentration, due to enhancing electrical mobility and carrier concentration. The improvement of electrical mobility was due to enhancement of crystallinity and increasing carrier concentration caused from generation of Ni metal sub-nano particles in NiO:AZO films after forming gas annealing. Meanwhile, the average optical transmittance was over 80% for all films except forming gas annealed 5.0 wt. % NiO:AZO films. Band gap widening (3.64 eV at 5.0 wt. %) was observed in annealed NiO:AZO films owing to the Burstein-Moss shift.
11:00 AM C4-1-10 Characteristics of Plasma Generated by ICP-CVD with Various H2/SiH4 Ratios and the Resultant Properties of nc-Si-H Thin Films
Jang-Hsing Hsieh, YuanLiang Lai (Ming Chi University of Technology, Taiwan, Republic of China); Chuan Li (National Central University, Taiwan, Republic of China); J. Setsuhara (Osaka University, Japan)
Nc-SiH thin films were deposited with an ICP-CVD system attached with four internal antennas, under the variation of H2/SiH4 ratios (R). During deposition, the generated plasma was characterized using a Langumir probe and an optical emission spectrometer (OES). The films’ properties were characterized using Raman spectrometry and FTIR. The results were correlated with those obtained from probe and OES studies. It was found that the crystallinity of nc-Si:H film was significantly affected by plasma density which was increased with the increase of R, but only to a certain extent. Both the plasma density and Xc reached the maximum at R=10, then leveled off. According to OES results, the ratio of IHa*/ISiH* also showed the same trend. Also, it was found that the crystallinity could be proportionally related to the increase of I*( SiH2+SiH3)/I(SiH+SiH2+SiH3) in FTIR spectra.
11:20 AM C4-1-11 Optoelectronic Characterization of p-type NiOx and n-type TiO2 Thin Films Deposited by Laser Ablation
Gustavo Hirata (CNyN-UNAM, US)

Nickel oxide (NiO) has been used in photoelectrochromic devices due to its p-type intrinsic behavior. Titanium oxide (TiO2) is an n-type semiconductor with excellent photocatalytic properties. In this work, Ni and Ti films were deposited by the PLD technique at room temperature and oxidized at 550ºC in air for 2 hours to form NiO and TiO2, respectively. The third harmonic (λ= 355 nm) of a YAG:Nd pulsed laser operated at an energy fluence of 3 J/cm2 and 10 Hz repetition rate was used for the laser ablation experiments. Microstructural development and chemical composition of the films were analyzed by SEM, XRD, AES/XPS and XRD techniques. Hall Effect and Van der Pauw measurements confirmed p- and n-type behavior of NiO and TiO2 films, respectively. Heterojunctions have been fabricated by deposition of p-type NiO on n-type TiO2 on ITO coated glass substrates. The relationship between electrical and photocatalytic properties of NiO and TiO2 thin films will be discussed.

Support from DGAPA-UNAM (Grant No. IN114010) and CONACYT (Grant No. 100555) and technical assistance by E. Aparicio, D. Dominguez and I. Gradilla are acknowledged.

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