PacSurf2014 Session TF-TuE: Thin Film Synthesis & Characterization I

ZnO is a remarkable multi-functional material with a distinctive property set and a huge range of existing applications. Recently, some research has been directed towards development of ZnO based materials with smaller band gap (Eg), which allow light emission/absorption over a broad spectrum from the UV to the visible region. One of the most promising means to narrow the band gap is to alloy ZnO with CdO that has small direct band gap of 2.3 eV. However, there are technological challenges for the growth of high-cadmium-content CdZnO because of the crystal phase segregation between wurtzite ZnO and rocksalt CdO.

In this context, we have developed a new compound, ZnInON (ZION), which is a pseudo-binary alloy of ZnO with InN. Since both ZnO and InN have the same wurtzite crystal structure and different band gaps (ZnO: 3.4 eV; InN: 0.7 eV), the alloy may possess band-gap tunability in a wide range. Here, we first demonstrate the sputtering synthesis of ZION and the band gap tuning by changing the chemical composition ratio. The detailed analysis on the crystal structure as well as the photo-electric properties of this new material are then conducted. Finally, we discuss the advantages of ZION as light-absorbing layers of multi-quantum well (MQW) solar cells, the theoretical efficiency of which exceeds 50%.

From the analyses of wide-range reciprocal space mapping of XRD spectra, we found that ZION has the same crystal structure as wurtzite ZnO and InN. The lattice constant of ZION is in between ZnO and InN, and decreases continuously with decreasing [Zn]/([Zn]+[In]), which indicate that ZION is a pseudo-binary system of ZnO and InN. The transmittance spectra reveal that the optical absorption edge is shifted continuously toward longer wavelengths with decreasing [Zn]/([In]+[Zn]). The band gap of ZION films, being determined by Tauc's plot, changes from 3.4 eV to 1.5 eV, which covers entire visible spectrum. The analyses of transmittance spectra indicate that ZION is of direct band gap, and thus it has high absorption coefficient around 10⁵ cm^-1. Furthermore, we found significant advantages of ZION in terms of applications in multi-quantum-well (MQW) solar cells. The lifetime of photo-generated carriers in QWs is increased by a few orders of magnitude when ZION is used as a MQW material instead of conventional GaAs-based materials, owing to the large piezoelectric field (several MV×cm^-1) that separate electrons and holes. These results indicate that ZION is a promising material that opens new pathways for realizing new generation solar cells with very high efficiency.

For improving the thermal stability of p-ZnO, we introduce Al to capture the N by forming Al-N chemical bonds. The bond dissociation energy, △H_f298, of Al-N is 297kJ•mol^-1, is much larger than the Zn-N（△H_f298 = 160 kJ•mol^-1). Theoretical calculations suggested the possible approach to the p-type doping in ZnO with III-V Ga-N co-doping or cluster-doping, which can increase the solubility and stability of N in ZnO. Here, we use Al to stabilize the N-doping, because the corresponding bonds with N and O are stronger for Al than for Ga. For stabilizing N in p-type ZnO, the key issue is to reduce the compensation of single Al doping due to the formation of only Al-O bands. Another crucial point is to make sure the Al captures more than one N-atoms to form AlN₂, AlN₃, and AlN₄ in ZnO. To realize that, we prepared Zn₃N₂:Al and convert it to ZnO by an oxidization procedure. For Zn₃N₂:Al, p-ZnO appears around 600 ^oC and do not convert back to n-type with further increase of annealing temperature. However, for Zn₃N₂, we did not obtain p-type ZnO. A reasonable explanation is that the Al will stabilize the N during the reaction to form ZnO from Zn₃N₂. Comparing with previous reports, the thermal stability is improved. However, the hole density is still low and needs further improvements. More details about the structural, optical, and electronic properties of the samples will be given in the presentation.

[1] R. Mitsuhashi, et al., Nature 464, 76 (2010)

[2] M. Yano, et al., J. Chem. Phys. 141, 034708 (2014)

[3] Y. Hasegawa, et al., J. Phys. Chem. C (in press)

Electrochromic (EC) thin films are important for enabling technologies such as smart windows and non-volatile displays. To date, various organic EC films have been shown to possess high coloration efficiency and many color variety, while inorganic films have demonstrated excellent optical modulation and cycling stability. In addition, flexible EC devices that are retrofittable to existing windows can also be achieved by utilizing organic or porous inorganic films, and this poses new challenges in device integration and performance enhancement. In this presentation, I will first talk about various synthesis methods used by us for EC thin film deposition. Solution-based methods which target for low-cost and large-scale fabrication will be emphasized. Besides chemical bath deposition and electroplating, an innovative dip coating method with better thickness control will be introduced. Materials covered range from inorganic metal oxides (such as nickel, tungsten, molybdenum and tantalum oxide) to organic polymer electrolytes. We found that different synthesis methods result in films with different structure, phase and defect density. These differences in turn significantly affect the electrochromic performance of the material. Therefore, in the second part of the presentation, I will focus on characterizing and correlating the physical and electrochromic properties of the synthesized films. As an example, layered nickel oxide/hydroxide will be discussed in details. With different synthesis methods and post treatments, we are able to investigate the electrochromic performance of films with different structures (compact, porous and nanostructured), phases (α and β phase), defect density (amount of Ni³⁺ sites) and intercalated species (water and anions). By changing electrolyte, we can also study the role of different ions in causing the color change, in particular, H⁺, OH^- and Li⁺. Recently, we have successfully identified the causes of the activation and degradation phenomenon observed in oxide material. We have also proved the importance of having existing reaction sites such as defects for improved electrochromic reaction involving Li⁺. The above knowledge can be extended to other layered material such as tungsten and molybdenum oxides. Last but not least, our ongoing work also focuses on synthesizing and characterizing thin films that can be functioned as solid electrolyte in electrochromic device. Besides the organic polymer electrolyte, we also look at emerging inorganic metal oxide electrolyte such as tantalum oxide. Their synthesis and ion conducting properties play significant role in device integration and performance.

Thin films composed of nanoscale particles possess unique and varied electrical, optical and mechanical properties, which can be used to create advanced functional materials and nanostructured composites for devices. Such thin films find applications in electronics, optics, photovoltaics, energy storage, biomedicine, etc.

Here we describe a general approach to nanoparticle thin film synthesis based on colloidal or nanogrinding. Nanogrinding can produce large amounts of nanoscale particles in an appropriate solvent without the use of complex chemical or physical processing. The nanogrinding method typically employs planetary ball milling to grind a starting bulk powder into a nanoscale colloidal suspension suitable for thin film coating. Ball milling uses the rotary motion of a grinding jar containing the sample, grinding beads and a liquid medium, which is arranged eccentrically on a so-called sun wheel. The efficient energy transfer during planetary ball motion leads to nanogrinding being a fast and inexpensive process for the creation of nanostructured thin films. In addition, nanogrinding is very flexible allowing the formation of semiconducting, metallic or insulating particles and films. These unique advantages combine to create a general solution-based processing approach for thin film surface coatings and devices (including non-planar geometries).

Results of nanogrinding trials using high-purity bulk materials (Si, silicides, oxides) and characterization of the nanoparticles and films via electron microscopy, scanning probe methods, Raman spectroscopy and elemental analysis are presented. Zirconia grinding vessels and either zirconia or silicon nitride beads are used in a planetary ball mill to grind readily available high-purity powders initially consisting of ~10-50 micron particles dispersed in solvent. Grinding speeds and times are varied between 200-1000 rpm and 10-500 minutes, respectively. After milling, the resulting colloidal dispersions are optionally filtered and deposited on a substrate for further analysis. As grinding time is increased, the resulting particles and films display good uniformity with sizes below 100 nm. Atomic force microscopy of the particles showed that diameters down to ~2-4 nm could be achieved. Elemental analysis indicates that contamination from the grinding tools is minimal.

Nanogrinding is an efficient route to directly creating nanoparticles suitable for thin film deposition of various materials from solution. By adjusting the grinding parameters, the size and dispersion of the particles can be controlled and their properties optimized for applications, including thin film electronic and optical devices.

Abstract:

The ever increasing demand for functionality and speed for semiconductor applications requires enhanced performance, which is achieved by the continuous miniaturization of CMOS dimensions. However, as we further reduce the device dimension, transistor with conventional structure and material is attaining its fundamental scaling limit. Beyond the 22 nm node fundamental as well as practical constraints will limit the maximum performance achievable by these scaled transistors. The integration of high-k gate dielectric provides a promising solution to continue improving the device performance, as their higher k-value allows a physically thicker layer while aiming the same capacitance and Equivalent Oxide Thickness (EOT). This study primarily focused on studying the electrical properties of high-k MOS capacitors with metal gate electrodes. In this work, we have deposited HfO₂ and Al₂O₃ thin film deposited on silicon substrate by RF sputtering technique and Plasma enhanced atomic layer deposition system (PEALD) respectively. The deposition of HfO₂ is confirmed by FTIR measurement. Current density vs voltage characteristics have been studied by I-V measurement further comparative study of current conduction mechanisms is done. In MOS structure due to the charge trapping and interface characteristics of dielectric films causes leakage current in dielectrics. The density of leakage current has been investigated and analyzed, by current-voltage and conduction mechanisms. Among the various conduction mechanisms HfO₂ and Al₂O₃ film on Si follows the Fowler Nordheim (FN) tunneling. The Poole Frenkel (PF) emission, Schottky emission (SE) and Trap assist tunneling (TAT) also studied. The barrier height (ф_B) is calculated of experimental work through Fowler Nordheim tunneling mechanism.

Keywords - HfO₂, Al₂O₃, high-k, leakage current density, FN tunneling.

High permittivity (high-k) dielectrics have been explored for an alternative gate insulator to conventional SiO₂ in the metal oxide semiconductor field effect transistor (MOSFET) technology. Among various high-k deposition techniques, atomic layer deposition (ALD) appears to be one of the suitable methods due to its excellent process controllability for extremely thin high-k films. Plasma-enhanced atomic layer deposition (PE-ALD) using plasma reactant has been spotlighted because of its several benefits over conventional thermal ALD using gas reactant, such as lower process temperature and improved film properties by the reactive radicals of reactants. However, use of plasma reactants negatively influences electrical properties of high-k films in MOSFET since ion bombardment of energetic radicals generates defects in films. Instead of radio frequency (RF) plasma which is usually used for plasma assisted processes, very high frequency (VHF) plasma for chemical vapor deposition (CVD) process was reported. The negative effects of plasma on film properties were reduced in VHF plasma process due to its lower ion kinetic energy and higher plasma density than RF plasma. Therefore, VHF plasma is expected to be a better plasma reactant for PE-ALD than RF plasma in the context of high growth rate and low ion damage. However, there has been no report on PE-ALD using VHF plasma reactant.

For this study, Al₂O₃ PE-ALD were developed using Trimethylaluminum (TMA) precursor and O₂ plasma reactant on Si substrate. O₂ plasma reactant was generated using two different frequencies, 13.56 MHz and 60 MHz for RF and VHF, respectively. The processes exhibited ALD mode with good self-saturation behavior and linear growth as a function of growth cycles. To investigate the characteristics of plasma density and electron temperature, a Langmuir probe was installed in the chamber. The thickness and density of the films was analyzed by spectroscopic ellipsometry and X-ray reflectivity (XRR), respectively. By using VHF plasma, better film properties were observed compared to RF plasma. The growth rate of VHF plasma (2.7 Å/cycle) was higher than that of RF plasma (2.3 Å/cycle). In addition, the VHF PE-ALD film was denser than RF PE-ALD film (3.26 for VHF and 3.11 g/cm³ for RF plasma), since ion and radical density generated by VHF is higher than those by RF. The VHF PE-ALD provides improved film qualities, such as low leakage current and low interface trap density, compared to RF PE-ALD grown film. The VHF plasma reactant for PE-ALD has potentials for various applications which require very thin and dense films in nanoscale regime.

We aim at developing waveguide devices that can transform the performance of the gradient echo quantum memory based on bulk crystals. Essentially this requires thin crystalline films of rare-earth doped oxides with low levels of impurities and defects. We therefore investigated the optimal conditions to prepare high quality Sc₂O₃ films on Gd₂O₃-buffered silicon wafers using pulsed laser deposition technique since (1) integration of the oxide on silicon will be compatible with standard semiconductor processing; (2) the oxides have minimum lattice mismatching with silicon; (3) there is a refractive index contrast between oxide and silicon in order to create the waveguide-based devices. Under the optimal conditions, X-ray theta-2theta scan of the film showed that, sharp oxide (222) peaks overlaid with silicon (222) and rocking curve of Sc₂O₃ (222) peak has a full width at half maximum(FWHM) of 0.10°. In-plain epitaxial relationship was confirmed by X-ray pole figure where Sc₂O₃ (111) is parallel to silicon(111). High resolution Transmission electron microscopy images indicated clear interfaces and perfect lattice images with sharp electron diffraction dots. All these confirm that the oxide films we deposited on silicon are single crystalline with high quality.