AVS2009 Session EM-TuP: Electronic Materials and Processing Poster Session

Wurtzute InN films were prepared on 6H-SiC substrate by self-designed plasma-assisted metal-organic molecule beam epitaxy system without buffer layer. In our report, we discussed the effects of substrate temperature on structural and optical properties of InN films. The crystalline and microstructure of the thin film was further characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), respectively. Electrical and optical properties were evaluated by Hall and photoluminescence (PL) measurements. XRD results indicated that InN films were polycrystalline and preferential grown along c-axis orientation. Two-dimensional growth mode was clearly shown from SEM images. However, cracks due to the lattice and mainly large thermal expansion coefficient mismatch were observed as well. Cross-sectional TEM images revealed that the InN films were grown continuously from the 6H-SiC substrate, and c-axis lattice constant was about 0.57 nm. Room-temperature PL spectra showed the emission peak is located at ~ 0.83 eV and sheet carrier concentrations is 7.9 × 10¹⁵ cm^-2. It was found that the optoelectronic properties and crystalline quality can be improved significantly by optimizing growth temperatures.

In this study, we discuss the correlations between electrical and optical properties of highly sp² amorphous carbon nitride for the development of electronic devices applications. The amorphous carbon thin films have been deposited using reactive plasma (Ar+N₂) radio frequency magnetron sputtering. The partial pressure of nitrogen has been used to vary the optoelectronics properties of the films.

The optical gap has been investigated using UV-visible-NIR spectroscopy and linked to the disorder parameter W^-1. The electrical conductivity has been studied in the temperature range of -170°C to 300°C and has revealed the presence of two conductivity modes. These results have been interpreted as a function of the density of states (DOS) evolution. The microstructural evolutions of the films with nitrogen incorporation have been investigated by Raman spectroscopy and IR absorption spectroscopy.

The disorder parameter W^-1 has shown a linear dependence with the optical gap in this range of materials (highly sp² carbon films) and has been interpreted as a measure of the overall disorder (structural and topological). At low nitrogen partial pressure, nitrogen incorporation promotes the graphitization and the clustering of the sp² phase. The preferential contribution of disorder has been identified as an increase of topological disorder (cluster size dispersion). The increase of electrical conductivity in this stage has been correlated to the increase of disorder and has revealed an increase of the sp² connectivity consistent with an increase of the cluster size.

The decrease of conductivity for the higher nitrogen content has been interpreted in a specific model for highly sp² materials (adaptation of Robertson’s two phase model). The proportion of CN triple bonds has been identified as a major effect on the material connectivity affecting the optoelectronic properties of the films.

Dye-sensitized solar cells have attracted attentions as next generation solar cells which have possibility to perform high efficiency with low cost. In recent years, various applications of the cells, such as colorful or flexible solar cells fabricated on polymer substrates, have been suggested. In conventional processes to form the TiO₂ photoelectrodes in the cell, the substrates should be annealed at higher temperature than 500 ^oC, which make it difficult to deposit TiO₂ films on polymer substrates. Sputter depositions should have advantages in fabricating the window-size uniform coatings of anatase TiO₂ at low temperature. In this study, dye-sensitized solar cells with TiO₂ photoelectrodes deposited by reactive magnetron sputtering on flexible substrates were fabricated and characteristics of the cells were investigated. A dual magnetron sputtering (DMS) system was used for the sputter deposition of TiO₂ [1-3]. This system consists of two magnetron cathodes with Ti metal targets, a dc power source with a 50 kHz pulse unit and plasma control unit (PCU) with a feedback system of plasma emission intensity (Fraunhofer Institut fur Elektronenstrahl-und Plasmatechnik, FEP). In the PCU, the plasma emission intensity of the Ti line at 500 nm was transformed into photovoltage (OEI) in order to control O₂ flow ratio. TiO₂ films were deposited on ITO/PET films and FTO glass substrates. Unipolar pulse or pulse packet modes were used for the sputter depositions. In order to control O₂ flow ratio in “transition” and “oxide” regions, oxidation of the target surface was precisely controlled using the feedback system. Sputtering power of each target was kept at 5 kW. Film thickness of TiO₂ was 3-10 μm. A sandwiched photovoltaic device was fabricated with N3-sensitized TiO₂ photoelectrode and Pt-coated glass as a counter electrode. The deposition rates of both the pulse modes were about 7 nm/min in the “oxide region” and about 40 nm/min in the “transition region”. The maximum value of conversion efficiency in this study was 3.7 % which was obtained in the cell with TiO₂ deposited with unipolar pulse mode in the oxide mode on FTO glass substrate. On the other hand, the cell with TiO₂ deposited in the pulse packet mode on ITO PET-films performed 1.25 %. Furthermore, we also recognized that TiO₂ with high photocatalytic decomposition activity showed high conversion efficiency where TiO₂ with poor photocatalytic activity showed poor conversion efficiency.

[1] S. Ohno, Y. Shigesato, et al., Thin Solid Films 445 (2003) 207.

[2] S. Ohno, Y. Shigesato, et al., Jpn. J. Appl. Phys. 43 (2004) 8234.

[3] S. Ohno, Y. Shigesato, et al., Thin Solid Films 496 (2006) 126.

The introduction of radiative electronic states within the bandgap of a semiconductor provide a mechanism for enhanced absorption of solar radiation and corresponding increase in short circuit current, while maintaining a large open circuit voltage. Approaches to introduce an intermediate band include the incorporation of quantum dots and doping of isoelectronic impurities. One promising material system is ZnTe (E_G=2.3eV), where the incorporation of oxygen provides a highly radiative state 0.4eV below the conduction band. In this work, the optical properties and photovoltaic response of ZnTe:O grown by molecular beam epitaxy on GaAs substrates will be presented. Photoluminescence measurements confirm a strong radiative transition for oxygen doping. Time-resolved photoluminescence measurements indicate a fast decay process from the conduction band, and a slow radiative decay from the oxygen states. Diodes consisting of ZnTe:O absorbers confirm that the response wavelength is extended to wavelengths beyond 900nm. Measurements consisting of two-photon excitation at wavelengths below the bandedge (650nm and 1550nm) further confirm transitions via intermediate band states. A device model for ZnTe:O intermediate band solar cells will be presented based on measured material parameters, and will be applied to determine both realistic and ideal conversion efficiencies attainable.

The electronic structure of the interfaces in dye-sensitized solar cell structures was investigated using x-ray and ultraviolet photoemission spectroscopy (XPS, UPS). Electrospray thin film deposition in high vacuum was used to build the interfaces of interest directly in vacuum without exposure to the ambient. Electrospray enables the fabrication of clean, essentially uncontaminated thin films of organic molecules and nanoparticles directly in vacuum.

The experiments focused on the investigation of the indium tin oxide (ITO)/nanocrystalline TiO₂ interface, as well as the characterization of the TiO₂/RuL₂(NCS)₂ [cis-bis(4,4’-dicarboxy-2,2’-bipyridine)–bis(isothio-cyanato)-ruthenium(II)] (“N3”, a prototypical dye used in many currently pursued device structures)-dye interface. Both TiO₂ and N3 films were built up in several steps. After each step, characterization by XPS and UPS was performed. The resulting sequence of spectra allowed the determination of charge injection barriers and interface dipoles at the ITO/TiO₂ and TiO₂/N3 interfaces. A particular focus of the experiments was the investigation of the influence of different surface conditions of the ITO on the electron injection barriers between TiO₂ and ITO.

Understanding vapor interactions with organic thin films is key to application of organic thin films in chemical sensing. Ultra-thin organic thin film transistors (OTFTs) fabricated using only 4 monolayers (4ML) of metal phthalocyanines (MPc) are model devices for studying sensing physics since analyte adsorption is almost entirely restricted to the air/MPc interface since adsorption within the grain boundaries is minimal. Even in ultra-thin MPc OTFT heterostructures the films are sufficiently thin that gas adsorption occurs primarily from interfaces instead of grain boundaries. In an ideal ultra-thin heterostructure, gas adsorption should occur only at the interfaces thereby creating carrier traps which alter the conduction in the OTFT channel. This was directly investigated using MPc heterostructure OTFT s. The response to isophorone doses for metal free phthalocyanine (H₂Pc) OTFTs and cobalt phthalocyanine (CoPc) OTFT s were compared to OTFT s fabricated with bilayer films (CoPc/H₂Pc and H₂Pc/CoPc). The sensitivity to isophorone is more than 5 times greater for H₂Pc OTFTs than CoPc OTFTs, and the desorption kinetics fit a single exponential decay for H₂Pc whereas a bi-exponential decay is required for CoPc. The heterostructure OTFT responses did not strongly correlate with the H₂Pc or CoPc OTFTs which suggests a combination of surface doping and adsorption at the H₂Pc/CoPc interface. This could lead to highly sensitive OTFT sensors based on multilayered MPc film structures.

Transparent conducting oxides (TCO) are the materials which combine visual transparency with high electrical conductivity. TCO films such as Indium Tin oxide (ITO) find its applications in photovoltaics, flat panel displays, electrochromic devices etc. ITO films are commonly grown by sputtering technique and presently meet current needs and quality for device applications. However, to achieve such good quality films, the films are grown at elevated substrate temperatures. This work explores a relatively newer vapor deposition technique known as pulsed electron deposition (PED) for the growth of ITO films where the films are deposited at room temperature. A commercially available target of ITO (90/10) was used as the source material. Films were deposited on soda lime glass and on Si (100) substrates. The oxygen pressure in the chamber during growth was varied from 2.8 mTorr to 22 mtorr. All the films were deposited for 5000 pulses. To evaluate the quality of grown films, various characterization techniques were employed. The optical transparency and the electrical conductivity of the films were found to be improving with increasing Oxygen pressure. Effect of Oxygen chamber pressure on resistivity, surface morphology, optical constants and carrier concentration on the films has been carried out. Details about the film preparation and evaluation of film properties will be presented.

We compare the charging response and defect generation, produced by vacuum ultraviolet (VUV) irradiation, of rapid-thermally annealed (RTA) 4nm thick HfO2 to as-deposited HfO2 on Si substrates. The HfO2 dielectrics were irradiated to 11.6 eV photons. The surface potential of the HfO2 samples was measured before and after VUV irradiation with a Kelvin probe system. The surface potential was determined to be negative before VUV irradiation and positive after irradiation for all samples except the 4nm thick HfO2 layer that was RTAd at 1000C. Paramagnetic defects within the HfO2 samples with and without VUV irradiation were measured with Electron Spin Resonance Spectroscopy (ESR). The VUV-irradiated samples indicate the presences of both E’ and Ex centers. From VUV-spectroscopy, the valence-band structure and location of defects with the band gap of the HfO2 samples were determined and compared to density of states calculations to determine the origin of the electronic states measured. Initial results from VUV-spectroscopy suggest the presence of oxygen-interstitial defects (OID) located within the HfO2 layer and oxygen-deficient Si centers within the SiOx interfacial layer. We show the electronic states of OID in HfO2 line up in energy with oxygen-deficient Si centers within the SiO2 interfacial layer. We believe the oxygen-deficient Si centers are responsible for the accumulation of positive charge in the VUV irradiated HfO2 samples. We conclude that charge exchange between OIDs within HfO2 and the oxygen-deficient Si centers within the SiOx interfacial layer is very important for controlling the radiation-induced trapped charge in HfO2/SiOx/Si dielectric stacks.

Supported by the National Science Foundation under Grant Number DMR-0306582 and the Semiconductor Research Corporation under Contract Number 2008-KJ-1781. The UW Synchrotron Radiation Center is funded by the National Science Foundation under Grant Number DMR-0537588.

Solid state thermoelectric cooling devices have been of current interest for hot-spot thermal management. Cooling hot-spots with high heat flux is becoming one of the most important technical challenges facing today’s IC industry. The rising temperature limits device minimization and decreases its lifetime. In this paper, we report to fabricate in-plane and cross-plane solid-state thermoelectric cooling devices using multilayered Bi₂Te₃/Sb₂Te₃ and Bi₂Te₃/Bi₂Te_3-xSe_x thin films. The Bi₂Te₃/Sb₂Te₃ and Bi₂Te₃/Bi₂Te_3-xSe_x multilayer thin films were deposited using sputtering deposition. The Bi₂Te₃/Sb₂Te₃ and Bi₂Te₃/Bi₂Te_3-xSe_x multilayer thin films have a periodic structure consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers or Bi₂Te₃ and Bi₂Te_3-xSe_x layers, where each layer is about 10 nm thick. The films were analyzed by XRD and SEM. The devices were fabricated using the standard integrated circuit (IC) fabrication process; pn junction diodes were fabricated as thermometers for the measurement of temperature in the devices. The fabricated in-plane and cross-plane multilayer thin film cooling devices and the achieved temperature difference from the cooling devices will be reported in the Conference. The developed devices could be a good candidate for the application of high-efficiency solid-state micro-cooling.

Directed assembly and subsequent photomodulation of anthracene-terminated phenylethynylthiolate molecules provide a means to control charge transfer in molecular switches. These fully conjugated molecules were selectively inserted as lone molecules, or in pairs, into defect sites of n‑alkanethiolate monolayers on Au{111}. Control of the assembly and a fixed molecular conformation on the surface allow a regioselective [4+4] cycloaddition between adjacent anthracene moieties under ultraviolet illumination. This photodimerization breaks the delocalized π network of the anthracene, which results in a dramatic conductivity decrease, observed as a photomodulated “off” state. The reaction between molecules also reduces stochastic conductance switching.