ICMCTF2017 Session CP: Symposium C Poster Session

Amorphous carbon nitride (a-CNx) films are known as useful coating materials. Recently, we first observed reversible photo-induced deformation of hydrogen-free a-CNx films under visible light illumination [1]. This phenomenon suggests that the a-CNx films have potential applications to light driven actuators. In this paper, we report fundamental studies for applying a-CNx films to light driven actuators.

The hydrogen free a-CNx films were prepared by reactive radio frequency magnetron sputtering using a graphite target and pure nitrogen gas. The substrate temperature during deposition was kept at 573 K. The substrates used were silicon single crystal plate with the thickness of about 0.5 mm. The self-standing a-CNx film with the thickness of about 1 μm was obtained by peeling the deposited film from the Si substrate in pure water. The diaphragm with the diameter of 4.6 mm was prepared by sandwiching the self-standing a-CNx film between metal rings and the movement of the diaphragm was measured using a laser vibrometer under white light illumination.

The photomechanical response was measured when the light illumination is turned on and off with the interval of 15 s, and the results show that the diaphragm reiterates stably and the typical amount of the displacement was about 120 μm. These results suggest that the a-CNx films have potential for light-driven actuators with good stability.

The authors would like to thank Keyence Corporation for helping the laser vibrometer measurements. This research is supported by JSPS KAKENHI Grant Number 26790054.

[1] M. Aono, T. Harata, N. Kitazawa, Y. Watanabe, Diamond & Related Materials 41 (2014) 20-24.

An innovative and scalable synthesis approach to the formation of Cu₂ZnSnS₄ (CZTS) nanocrystals has been developed using aerosol spray pyrolysis. This quaternary-phase material is a potential replacement for currently commercialized semiconductors such as CdTe and CIGS that are currently used in photovoltaic devices. However, sustainability and environmental issues threaten long-term viability of these materials. Based upon earth abundant constituents and low chemical toxicity, CZTS, with a reported band gap of ~1.5 eV, appears to be a superior alternative to these other materials. Additional research and development is necessary to increase the efficiency of CZTS-based cells from the current record (12.6% by Wang et al.^[1]) to the >18% necessary to be considered commercially viable. Our work stresses the controllable, cost-effective, and reproducible synthesis of high-quality CZTS nanoparticles and sintered thin films. Specifically, our goal is to demonstrate increased crystal grain growth in CZTS needed for higher efficiency rates of CZTS-based cells. Incorporating alkali dopants, specifically sodium, has been successful in enhancing grain growth in CZTS thin films ^[2]. However, current methods of sodium doping primarily rely on imprecise external diffusive sources such as sodium halides, sodium hydroxides, or the use of soda lime glass. Based off of work by Tiong et al., who experimented with controllably doping sodium into CZTS films via dipping in sodium halide solutions ^[3], we show further results of attempting to control sodium composition by coating the surface of CZTS nanoparticles with a controlled amount of sodium hydroxide – our group has also shown that the introduction of an oxide to the surface of CZTS nanoparticles can also enhance uniform grain growth. The coated particles are then dispersed into a CZTS nanoparticle ink, coated onto a sodium-free substrate, mechanically compacted, and annealed at 600 ºC for 1 hour in a low pressure sulfur atmosphere. The resulting material is extensively characterized to determine morphology, composition, and structure.

^[1] Wang, Wei, Mark T. Winkler, et al. “Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency.” Advanced Energy Materials 4, no. 7 (2014).

^[2] Johnson, M., S. V. Baryshev, et al. “Alkali-Metal-Enhanced Grain Growth in Cu2ZnSnS4 Thin Films.” Energy & Environmental Science 7, no. 6 (2014): 1931-38.

Amorphous oxide semiconductors (AOSs) have demonstrated their advantages as applications for flat panel displays (FPDs) due to their electrical performance, transparency in visible light, and room-temperature deposition. Oxygen vacancies in AOS materials play the role of generating carriers and thus provide a conducting path and at the same time, they are components that may deteriorate the stability of the AOS-based thin film transistors (AOS-TFTs) by forming defects. To improve the stability of AOS-TFTS, the oxygen vacancies have to be reduced. Recently, a number of studies regarding the incorporation of materials with a low standard electrode potential (SEP) into AOS materials, such as yttrium (Y), hafnium (Hf), and zirconium (Zr), have been reported. Particularly, Hf has a lower SEP (- 1.56 eV) than that of Zn (- 0.76 eV) and HfO₂ has a larger bandgap (5.8 eV) than that of ZnO (3.3 eV). Thus, the addition of Hf to the indium-zinc-oxide (IZO) system is expected to control the electrical characteristics of the films and improve the stability of TFTs by controlling oxygen vacancies. Recently, K. Ghaffarzadeh et. al. introduced TFTs using hafnium-indium-zinc-oxide (HIZO) as an active layer. They showed that the stability of the HIZO TFTs due to bias-stress and photo-stress was improved as a result of the reduced interface charge trapping. However, when Hf is used in TFTs, it may decrease the channel mobility due to the reduction of carrier concentration, leading to the significant decrease of the on-current. Furthermore, the In element used in the HIZO TFTs is an earth-rare material. For this reason, some studies without using In in AOS-TFTs have recently been reported to aim at maintaining high channel mobility.

In this work, (Al, Co)-ZnO films were co-sputtered on glass substrate through radio frequency sputtering at 100 °C. The film’s structure, electrical and magnetic properties as a function of Al doping content is investigated. The results indicate that (Al, Co)-ZnO films crystallinity can be suppressed by Co doping or (Co, Al) co-doping. With the substitution of Zn by Al , the film’s conductivity improves. All the films present ferromagnetic behavior at room temperature. With increasing Al doping amount, the film’s saturation magnetization expresses a carrier-concentration dependent behavior. Three different regions can be defined, where BMP model and carrier-mediated exchange mechanisms play a role in the various regions.

Atomic layer deposition (ALD) is unsurpassed as a technique for application of uniform, conformal, and continuous thin films on a broad range of useful substrates, especially those with ultra-high aspect ratios or complex morphologies, as the precursors are in the gas phase and easily fill all spaces with characteristic lengths greater than 0.5 nm. Three principal shortcomings of conventional ALD include: (1) reliance on vacuum pumps to reduce the pressure in the growth chamber and pull gas in resulting in long individual half cycle times—many of which are required to apply films of appreciable thickness (~50 nm), (2) contamination from precursor gas reactions that only proceed to a fraction of total completion, and (3) challenges (high temperature, high vacuum, containment within vacuum reaction vessel, etc.) associated with integration into additive manufacturing which could lead to custom components with integrated antennas and other device functionality especially in components employing conformal nature of the ALD coatings. Novel atmospheric plasma-based processes for synthesis of semiconducting oxides such as ZnO, GaO, IGZO (indium gallium zinc oxide) are presented. Techniques include a rapid ALD process where gas is cycled thru microvolumes adjacent to the substrate, rather than filing and vacating an entire processing chamber with the necessary series of vapor phase precursors to increase the effective growth rate of materials substantially. The precursors are selected so gaseous byproducts after ALD film formation are safe for exposure to open air. Results from optical spectroscopy of the atmospheric plasma processes is presented to demonstrate relationships between process characteristics and the structure and composition of the materials. For example, the degree of ZnO crystallization will play a critical role in its transport properties, and will therefore be thoroughly characterized using Raman spectroscopy and X-ray diffraction. The composition will first be measured with XPS, which has a resolution of approximately 1 atomic percent. Once contaminants are below this level, property measurements will be most useful for understanding atomistic-scale defects such as impurity atoms or vacancies. Nanocrystalline ZnO grown via pulsed laser deposition is a semiconductor material reported to possess high charge mobility (>100 cm² V^-1 s^-1), an extremely high on/off ratio (>10¹²), and be useful for high frequency (500 MHz) device operation. These parameters set the baseline for our comparisons of atmospheric pressure plasma enhanced ALD, where the aim is to match or surpass these values of mobility, on/off ratio, and maximum operating frequency.

The use of rare earth elements with semiconductor materials has attracted a lot of interest due to its unique properties. In this paper, we investigated ytterbium(Yb)-doped zinc tin oxide (Yb:ZTO) thin film characteristics and its application to a potential down-converter of Cu(InGa)Se2 (CIGS) thin-film solar cells. Yb:ZTO thin films were deposited by reactive sputtering of Zn and Sn metal with an oxygen flow. A few pieces of Yb were embeded in Zn metal target and thus Yb elements were supplied during Zn sputtering process. The relatively composition of Zn and Sn was controlled by changing the sputtering power(10-70W) of Sn with the fixed sputtering power for Zn(70W). Also, the substrate temperature varied from room temperature to 400 degC. It was confirmed that smaller amount of Sn with lower sputtering power led to more incorporation of Yb into ZTO. The X-ray photoelectron spectroscopy analysis confirmed that incorporation of Yb in ZTO, and photoluminescence measurement demonstrated Yb emission. The glazing incidence X-ray diffraction showed the shift of ZTO peaks induced by the difference in composition of Zn and Sn. Finally, CIGS solar cells with a Yb:ZTO layer have been fabricated. The results suggested that cells with the highest Yb PL emission showed the highest short circuit current density and cell efficiency.

The Molybdenum (Mo) has been used as a back-contact electrode for high-performance chalcopyrite Cu(InGa)Se2 (CIGS)-based thin film solar cells, which were recently reported to have the world-record cell efficiency of 22.7 % (ZSW, 2016). The substrate-type typical CIGS cell structure is glass/Mo/CIGS/CdS/ZnO. In this paper, to investigate the feasibility of CIGS cell for the application to bifacial solar cells, Mo layer has been patterned and a few layers of transparent conducting graphene were inserted to glass/Mo interface yielding glass/graphene/Mo(patterned) substrate. The graphene sheets grown on Cu foil by chemical vapor deposition were transferred to glass substrates by a simple wet-based graphene transfer process. Then, Mo was deposited onto glass/graphene by DC sputtering. The diverse design of Mo patterning for light transparency was achieved by using custom-designed masks during Mo deposition. It was confirmed that glass/graphene/Mo thin films showed lower sheet resistance than glass/Mo samples, and the sheet resistance was monotonically decreased with the number of graphene layers inserted. The resulting CIGS device results will be also discussed.

Taking advantage of the high impermeability property of graphene to gases and liquids [3], we demonstrate that a graphene on metals substrates can be used to prevent it from chemical reactions and degradation of the adhesion of the metal deposition over the glass substrates. Raman spectroscopy has been used to verify the existence and quality of graphene after transfer process over gold and silver thin films.

The stability measurements where performed in atmosphere, and are based on the monitoring of SPR angle and full width half maximum (FWHM) during time.

Our results demonstrate that graphene protected gold (Au) and silver (Ag) depositions exhibit greater stability as compared to unprotected samples (exposed to air), similar to the one associated to samples protected by typical OLEDs encapsulation technique [4]. We observed a shift in the SPR angle of about 0.005⁰ for Ag /graphene and Au/graphene samples after 4 hours of observation. Such a change is comparable to the zero angle determination and limits the time interval useful for the characterization of thin films in air. Similar results were obtained for graphene covered dielectric loaded waveguides (DLWGs).

The presented stability enhancement is very important both for the graphene optical characterization [5] as well as to the use of graphene in biosensing application [6].

1. V. G. Kravets, R. Jalil, Y.-J. Kim et al., Scientific Reports, 4 (2014) 05517.

2. X. Li, L. Colombo, R. S. Ruoff, Adv. Mater. 28 (2016) 6264.

3. Y. Su, V.G. Kravets, S.L. Wong, J. Waters, A.K. Geim, Nature communication 5 (2014) 4843.

4. T. Del Rosso, J. E. H. Sanchez, R. Dos Santos Carvalho, O. Pandoli, M. Cremona, Optics Express 22 (2014) 18915.

5. H. Jussila, H. Yang, N. Granqvist, Z. Sun, Optica 3 (2) (2016) 151–158.

6. O. Salihoglu, S. Balci, C. Kocabas, Appl.Phys. Lett. 100 (2012) 213110.

Metal-Oxide-Semiconductor (MOS) structures using SiO_x films as gate insulators are promising for application in different types of optoelectronic devices. In this work we present results for the effect of UV light on the electrical characteristics of MOS structures containing crystalline or amorphous Si nanoparticles in the gate insulator.

SiO_x films with thickness of ~50 nm and various compositions, x varies between 1.15-1.5, were deposited by thermal evaporation in vacuum on n-type crystalline Si. After the deposition the samples were annealed at temperatures in the 250 - 1000 ^oC range for 30, 60 and 120 min. High temperature annealing at T ≥ 700 ^oC leads to formation of amorphous and/or crystalline Si nanoparticles in SiO_x layers. The formation of nanocrystals in the samples annealed at 1000 ^oC was verified by Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). High resolution TEM images revealed that the nanocrystal size depends on the composition and the annealing time; for example, samples with x = 1.15 and 1.3 annealed at 1000 ^oC for 60 min exhibit nanocrystals with diameters of approximately 4 and 6 nm, respectively. The XPS results showed that the annealing at 1000 ^oC leads to complete phase separation and formation of Si in SiO₂.

Lateral currents between two metal contacts on the top of the Si/SiO_x structure were measured in dark and under visible and UV light illumination. The dark and visible light I-V characteristics coincide, while the UV light leads to an increase of the current through the structure. The UV effect is more pronounced with the increase of the annealing temperature. The observed effect may be explained assuming UV light assisted transport between neighbor nanocrystals of electrons injected from the crystalline silicon wafer.

The obtained results indicate that the studied SiO_x layers have a potential for application in UV sensors.

The nanocomposites of zinc oxide/graphene oxide (ZnO-GO) were synthesized to fabricate the photodiodes. The ZnO-GO/p-Si and ZnO-GO/n-Si diodes were prepared for various GO contents. The electrical characteristics of the ZnO-GO/p-Si and ZnO-GO/n-Si diodes were analyzed under dark and light illuminations. The photocurrent of ZnO-GO/p-Si and ZnO-GO/n-Si diodes increases with increasing GO content.