AVS2009 Session TF-ThP: Aspects of Thin Films Poster Session

Technological solutions for producing nanoscale cermet resistor films with sheet resistances above 1000 Ω /□ (Ohm per square) and low temperature coefficients of resistance (TCR) have been investigated. 2 to 40 nm thick films were sputter deposited from CrSi₂-Cr-SiC targets by a dual cathode dc S-Gun magnetron. In addition to studying film resistance versus temperature using four point probe measurements, scanning electron microscopy and atomic force microscopy were also employed for analysis of the nanofilm structure features. This study has revealed that the cermet film TCR displays a significant increase when the deposited film thickness is reduced below 2.5 nm. An optimized sputter process consisting of wafer degassing, cermet film deposition at elevated temperature with rf substrate bias, and a double annealing in vacuum consisting of an in situ annealing following the film sputtering and an additional annealing following the exposure of the wafers to air has been found to be very effective for the film thermal stabilization and for fine tuning the film TCR. Cermet films with thicknesses in the range of 2.5 - 4 nm deposited using this process had sheet resistance ranging from 1800 to 1200 Ω /□ and TCR from – 50 ppm/˚C to near zero, respectively. A possible mechanism responsible for the high efficiency of annealing the cermet films in vacuum (after preliminary exposure the films to air) resulting in resistance stabilization and TCR reduction is discussed.

Diamond-like carbon (DLC) has the amorphous structure that is chiefly composed by graphite (sp²) and disordered graphite (sp³) state. Therefore mechanical property of DLC generally shows high hardness and low friction. DLC film has been prepared by various method of chemical vapor deposition (CVD) or physical vapor deposition (PVD) including the sputtering method. Commercial application of DLC has been already performed as engine parts of an automobile or surface coating of a hard disk.

In this study, DLC films were formed using Ar⁺ or He⁺ ion beam assist in a naphthalene (C₁₀H₈) atmosphere. C₁₀H₈ is aromatic hydrocarbon with two benzene rings of a solidly in normal temperature and pressure. In our previous study, toluene (C₇H₈) of aromatic hydrocarbon with one benzene ring was used at same experimental process. By using C₁₀H₈ as an atmosphere gas, the higher deposition rate is expected than C₇H₈ gas. The formation conditions of DLC were changed with ion beam accelerating voltage and current density. Current dependence was performed by ion beam current density of 10 to 70 μA/cm² with a constant accelerating voltage of 5 kV, while voltage dependence was performed by 1 to 12 kV with a constant current of 10 μA/cm².

The mechanical properties of hardness and friction coefficient were determined using the dynamic micro Knoop hardness tester and the pin-on-disk tribo-tester with a SUJ2 ball of a 1/4 inches diameter, respectively. The conditions of examination were fixed at a load of 0.98 N, a revolution speed of 135 rpm, a sliding diameter of 10 mm and a sliding distance of 10 m. Atomic concentration and structure of the films were investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, respectively.

The suitable property of Knoop hardness and friction coefficient were obtained by conditions of accelerating voltage of 5 kV with current density of 10 μA/cm². Knoop hardness of the film showed 5.37 GPa using Ar⁺ ion beam irradiation, while the friction coefficient of the film showed 0.117 using He⁺ ion beam irradiation. It was clear that property of DLC film was changed by ion species. In the case of He⁺ ion beam irradiation, low friction property was shown at 5 to 7 kV with 10 μA/cm², while high hardness property was obtained by Ar⁺ ion beam irradiation of 5 kV with 10 to 40 μA/cm².

Amorphous carbon-Au (a-C:Au) composite thin films were synthesized by co-sputtering using a graphite target with an attached piece of pure gold. Under this configuration and depending on the deposition conditions and materials, it is possible to produce different structures; multilayers; nanocomposite thin films, or compounds. The aim of the work was to determine if by analysing the ellipsometric spectra of samples deposited on silicon substrates it would be possible to distinguish between the different structures. For this purpose, different deposition powers (40 to 130 W) were investigated, keeping the other deposition conditions. The film microstructure and composition were evaluated using X-ray diffraction (XRD) and Rutherford Backscattering (RBS).

As deposited films did not show any characteristic signal in the ellipsometric spectra related to the presence of gold, in agreement with the XRD results where no diffraction peaks were observed. Nevertheless, the RBS data showed that the gold concentration could be varied between 9 and 7 at% as the power increased. The samples were submitted to a thermal treatment in an Argon atmosphere at 600 °C, in order to promote the gold segregation. Ellipsometric spectra after the annealing clearly showed the absorption related to the interband transitions of gold (~ 2.5 eV). The spectra for each deposition power were modelled to obtain the variations in the optical properties of the films due to the gold incorporation, either as atomic inclusions (as-deposited samples) or particles (annealed samples) using Tauc-Lorentz models for the carbon matrix and the extended Drude-Lorentz dispersion model for the gold particles.

Magnetron sputtering processes using ceramic oxide targets have been used to deposit transparent conductive Al doped ZnO films because of its advantages for large area uniform coatings with high packing density and strong adhesion. However, the degradations of electrical properties and crystallinity for ZnO films have similarly been observed at the positions opposite to the erosion track on the target. These are considered to be caused by the bombardment of high energy particles such as energetic Ar atoms (high energy neutrals) or negative oxygen ions. In this study, we tried to detect the flux and energy distributions of high energy negative ions during the dc magnetron sputtering using an AZO target and discussed the influence of high energy negative ion bombardments on the structure and electrical properties of the films.

High energy negative ions were analyzed using a quadrupole mass spectrometer combined with an electrostatic energy analyzer, which was positioned at the substrate position opposite to the AZO (Al₂O₃: 2.0 wt%) target. The sputtering power during the analyses was maintained at 50 W. The O₂ flow ratio [O₂ / (Ar+O₂)] were controlled from 0 to 5 %. For the analysis of the flux of the negative ions at the different substrate locations, the sputtering target was perpendicularly moved to the quadrupole mass spectrometer. In order to control the cathode voltage, the magnetic field strength was selected as 0.025, 0.06 and 0.1 T. In order to discuss the influence of the bombardments on the film properties, AZO films were deposited on unheated alkali-free glass substrate under the same condition of the fragment analysis. The atomic oxygen negative ion (O^-) was observed as the high energy negative ions which possessed the energy corresponding to the cathode sheath voltage. The maximum flux of O^- was observed at the location opposite to the erosion track on the target. The flux of O^- decreased slightly with increasing O₂ ratio. These results indicate that high energy negative ions were not formed by electron attachment in the cathode sheath region but should be sputtered from the target surface. Depending on the magnetic field strength, the cathode voltage varied from 337 V at 0.1 T to 403 V at 0.025 T. While the peak of O^- shifted to lower energies with increasing the magnetic field strength, the flux of O^- was hardly changed. The lower the energy of the peak of O^- which AZO films is deposited at is, the lower both of resistivity and crystallinity for AZO films which are deposited at the positions opposite to the erosion track on the target are.

DLC (Diamond-like Carbon) classified in new materials is amorphous carbon including hydrogen and has the similar property to diamond. DLC film was formed by the ion beam evaporation method in the early 1970's and after that has been manufactured by various methods such as CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition). Because the representative mechanical property of DLC shows the high hardness and low friction coefficient, DLC is applied in various fie lds such as motor parts or tools. Also the electric property of DLC is anticipated as material of a field emission source.

In this study, mechanical properties were investigated about Si- doped DLC thin films prepared by using ion beam and electron beam evaporation in a C₁₂H₂₆ atmosphere . The formation of DLC thin films was performed by assist of He⁺ ion-beam irradiation in a C₁₂H₂₆ atmosphere and doping of Si into DLC was performed by electron beam evaporation. In this experiment, Si concentration in DLC was changed by control of an electron-beam evaporation source, while He⁺ ion beam was irradiated at a constant accelerating voltage of 5 kV with a current density of 20 μA/cm². Film composition and microstructure were investigated by X-ray photoelectron spectroscopy and Raman spectroscopy. The hardness was measured from an indentation method with a Knoop indenter. The friction coefficient was measured for an SUJ2 ball with a constant load of 0.98 N until the sliding distance reached to a length of 100 m.

The improvement of mechanical properties for Si-doped DLC thin films was exhibited in this experiment. Knoop hardness of the DLC thin film with Si concentration of about 40 percent showed 8.2 GPa. Friction coefficient of the DLC thin film with Si concentration of about 24 percent showed 0.107 at sliding distance of 100 m. Each property corresponds to increase of about 46 percent in hardness and decrease of about 74 percent in friction coefficient as compared with un-doping DLC thin film prepared with an accelerating voltage of 5 kV at a current density of 20 μA/cm².

Since the photoinduced decomposition of water on TiO₂ electrodes was discovered, semiconductor based on photocatalyst has attracted extensive interest. TiO₂ is anticipated as one of materials which are alternative for existing solar cell technology based on silicon. TiO₂ shows relatively high reactivity and chemical stability under UV light whose energy exceeds the band gap of 3.2 eV in the anatase crystalline phase. The sun as an energy source can provide an abundant photons, however, UV energy in the sunlight accounts for only a small fraction (～5%) compared to the visible region (45%). Many techniques have been examined to achieve the purpose, i.e. harness of the visible light. Improvement of TiO₂ has been performed by doping transition metals or anionic species, but these doped materials induce thermal instability and an increased number of carrier recombination centers. On the other hand, the relationship between structure and photofunctional properties of TiO₂ has some unclear points. Different TiO₂ structures can be obtained in the reactive magnetron sputtering method by control of O₂ gas flow rate and formation temperature.

In this study, TiO₂ was prepared by reactive magnetron sputtering using Ti target in an Ar/O₂ gas mixture. Composition and microstructure of these TiO₂ films were investigated by XPS and XRD, respectively. The surface morphology of TiO₂ was observed by AFM. Chromatic change of a methylene blue solution was applied as photofunctional property. Light irradiation to TiO₂ in a methylene blue solution was carried out using a commercial sterilizing lamp as UV light and an artificial sunlight lamp (with UV filter) as visible light. Transmittance of a methylene blue solution was measured by a spectrophotometer.

As results of XRD, the crystal structure of TiO₂ turned from a ruttile type into an anatase type by each increase of O₂ gas flow rate and formation temperature (100~300℃). In addition formation temperature had a large effect on TiO₂ surface morphology and roughness. The TiO₂ film prepared with high formation temperature showed a smooth surface. In the case of 300℃ in formation temperature, the higher photofunctional property under irradiation of UV light was obtained at an anatase type (O₂ gas flow rate of 2.5 sccm). In the case of visible light, lower photofunctional property was shown as compared with the case of UV light. However, photofunctional property showed the maximum value at a ruttile type (O₂ gas flow rate of 1.0 sccm) in visible light. The mutual relationship between the photofunctional property and the formation condition was dependent on not only the film structure but also the surface morphology.

In this study SnO₂ films doped with Sb or Ta (ATO or TTO, respectively) were deposited on unheated or heated glass substrates at 200^oC by the reactive sputtering with Sb-Sn or Ta-Sn alloy targets using a plasma control unit (PCU) and mid-frequency (mf, 50kHz) pulsing. PCU feedback system (Fraunhofer Institut fur Elektronenstrahl-und Plasmatechnik, FEP) monitors the oxidation states of target surface by observing in cathode voltage (impedance control method) [1]. The mf pulsing possesses the approximate shape of a square wave which make it possible to reduce arcing on the target when high power density is applied. In the case of the ATO films deposition on heated substrate at 200^oC in the “transition region”, the deposition rate was 280 nm/min where the lowest resistivity of the ATO films was 4.6×10^-3 Ωcm and the optical transmittance was more than 80% in the visible region.

[1] M. Kon, Y. Shigesato, et. al, Jpn. J. Appl. Phys. 41, 814 (2002).

Transparent conductive oxide (TCO) is a highly degenerated wide band-gap semiconductor with low electrical resistivity and high transparency in the visible and near-infrared regions. In this study we will report the very high rate deposition of various TCOs, such as Al-doped ZnO (AZO), Sn-doped In₂O₃ (ITO) or Sb-doped SnO₂ (ATO) films by reactive sputtering using Zn-Al, In-Sn or Sn-Sb alloy targets, respectively.

In general the deposition rate for the sputtering using the oxide ceramic targets is not so high and also the cost for the high quality ceramic targets is high. On the other hand, reactive sputtering using the alloy targets should be one of the most promising techniques to achieve much higher deposition rate for various industrial applications because sputtering yield of the metallic surface is much larger than oxide surface and also the higher sputtering power density can be applied for metallic targets with the higher thermal conductivity. The reactive sputtering process, however, is strongly affected by the O₂ flow ratio; the deposition rate exhibits hysteresis with respect to the O₂ reactive gas flow rate. Such behavior originates in the oxidation state of the target surface, resulting in the marked decrease in deposition rate with the increasing O₂ flow. Therefore, the sputtering conditions should be precisely controlled so as to obtain high-quality TCO films by reactive sputtering processes with a high deposition rate and with high reproducibility. In order for the precisely controlled deposition a specially designed feedback system (Fraunhofer Institut fur Elektronenstrahl-und Plasmatechnik, FEP) of discharge impedance or plasma emission intensity combined with mid-frequency (mf, 50 kHz) pulsing has been carried out [1-5] . Oxidation of the target surface was precisely controlled by these feedback systems in the “transition region”, where the deposition rate and the stoichiometry . The deposition rate was about 10-20 times higher than the one deposited by conventional sputtering depositions using oxide ceramic targets.

[1] M. Kon, Y. Shigesato, et al., Jpn. J. A ppl. Phys. 41, (2002) 814.

[2] M.Kon, Y. Shigesato, et al., Jpn.J. Appl. Phys., Vol.42, No.1, (2003) 263.

[3] S. Ohno, Y. Shigesato, et al., Jpn. J. Appl. Phys., Vol. 43, No.12 (2004) 8234.

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

[5] S. Ohno, Y. Shigesato, et al., Science and Technology of Advanced Materials 7 (2006) 56.

TiO₂ as photo-functional material is one of lower cost material and harmless material to environment. It is expected to use as material of clean energy in future. Furthermore the photocatalytic property provides antibacterial or antifouling effect. These effects decompose environmental pollution matters (like nitrogen oxide etc.) by generate active oxygen (O^2-, OH), when TiO₂ is exposed to sunlight. On the other hand, TiO₂ has characteristically electrical properties such as an n-type semiconductor or a dielectric. In order to improve the electrical and photocatalytic property of TiO₂, many researchers have used various methods such as gas or metal doping into TiO₂ techniques etc. In this study, tungsten (W) was doped to TiO₂ thin film to improve the electrical properties and to enhance the photo-sensitivity of TiO₂ thin film. The doping of W in TiO₂ thin film generates tungsten oxide (WO₃) and this oxide shifts the conduction band of TiO₂ to the positive side because of the low band gap of WO₃ (2.8eV) as compared with that of TiO₂ (3.2eV). As a result, because the motion energy of each electron becomes smaller, excited electrons in a visible region (wavelength more than 400 nm) increase and promote the photocatalytic reaction in this region.

TiO₂ thin films were prepared by using the reactive magnetron sputtering method on stainless steel substrate (SUS304) of 18×18 mm in size and also glass substrate of 18×9 mm in size. All samples were formed in four layers. The first one was a Ti layer with 50 nm in thickness. The second layer was a TiO₂ layer of 170 nm. The third layer and surface layer were W-doped TiO₂ of 30 nm and TiO₂ of 10 - 60 nm, respectively. Other formation conditions were 1.1 sccm in O₂ gas flow rate and 20 sccm in Ar gas flow rate. Ti and W sputtering rate were fixed at 0.025 nm/sec and 0.002nm/sec, respectively. The substrate temperature through this formation process was set at 200 ^oC.

The photocatalytic property was measured by a methylene blue immersion test. The difference in light absorbance at a wave length of 665 nm after light irradiation for 12 hours using sterilization, fluorescent and an artificial sunlight lamp (with UV band filter) was measured by a spectrophotometer (SHIMADZU UV-2550). Photoelectric current was measured by a cyclic volt ammeter system.

The photo-functional properties of W-doped TiO₂ were improved by the additional TiO₂ onto the W-doped TiO₂ layer. Photocatalytic property showed a higher value under artificial sunlight irradiation when the surface layer thickness was 20 nm.

There has been a growing interest in the dielectric material that can be deposited at low substrate temperatures for the applications such as organic devices and flexible display devices. Of diverse dielectric materials, silicon-nitride film has been widely used for various important applications from semiconductor to flat panel display, such as a gate dielectric material for thin film transistor (TFT), passivation layers for diverse microelectronics and as anti-reflection (AR) coating for solar cell. In addition, due to their chemical inertness, excellent dielectric properties, and thermal stability compared with those of silicon oxide, many researchers have been studied to develop high quality low temperature silicon-nitride films using various types of plasma sources. Generally, conventional technique for depositing SiN at a low temperature is plasma-Enhanced CVD (PECVD) (~300℃).

In this study, we carried out the deposition of silicon nitride thin films at the temperature lower than 100℃ by using an internal linear ICP source. To obtain high quality silicon-nitride films, the effects of the ratio of NH₃ to SiH₄ and DC bias on the properties of thin film were investigated. The results showed that, by using 2:1 ratio of NH₃:SiH, and by using -150V DC bias, the high quality silicon nitride film having the refractive index of 1.83, dielectric constant of 7.2 with negligible interface traps could be observed. The compositions, binding states, and the refractive indices of the films were measured using a XPS, FTIR, and an ellipsometer, respectively. In addition, metal/insulator/semiconductor (MIS) capacitors having Al/insulator/p-Si were fabricated and the flat-band voltage and hysteresis voltage were measured by the capacitance-voltage (C-V) method.

In a traditional glancing angle deposition (GLAD) process, the column tilt angle β and film density ρ are both governed by the deposition angle α. It was later discovered that β could be controlled independently from the deposition angle by implementing an appropriately designed phisweep substrate rotation algorithm which reduces anisotropic shadowing¹. Ion-assisted deposition has also been demonstrated to alter columnar thin film morphology² which has proven to be useful in humidity sensing applications³. The influence of the phisweep process on β is inherently limited to producing tilt angles that are less than or equal to that of a traditional GLAD film grown without phisweep. Ion assisted GLAD is a procedure that can increase β above the maximum achievable tilt angle in a standard GLAD deposition and has been utilized in square spiral photonic crystal fabrication⁴. The work reported here is a fundamental study of SiO₂ columnar thin films grown with an ion-beam assisted GLAD process. Our current capabilities in modifying β as a function of α with an ion assisted process will be described. In addition, we report on our efforts to decouple the film density ρ from α; a previously unstudied characteristic of the ion assisted GLAD process. This latter result could enable partial decoupling of α, β and ρ simultaneously and allow access to previously unattainable columnar film morphologies, improving the versatility of the GLAD process. Discussion will focus on the effect of ion assistance on ρ and β as measured by variable angle spectroscopic ellipsometry (VASE) and the influence of different α. The relationship between these parameters, the natural column broadening and effects on the resulting in-plane birefringence will also be discussed.

[1] M. O. Jensen, and M. J. Brett, Appl. Phys. A, 80, 763 (2005)

[2] I. Hodgkinson, and Q.H. Wu, Mod. Phys. Lett. B, 15, 1328 (2001)

[3] M.T. Taschuk, J.B. Sorge, J.J. Steele, and M.J. Brett, IEEE Sensors Journal, 8, 1521 (2008)

[4] J.B. Sorge, M. A. Summers, M. D. Fleischauer, K. Tabunshchyk, A. Kovalenko, and M. J. Brett, Mat. Res. Soc. Sym. Proc., 1014E, 1014-AA07-26 (2007)

We present a study of the relationship between structural properties and opto-electronic quality of aluminum doped zinc oxide produced by rf magnetron sputtering. Thin films (300-400 nm thick) were deposited as a function of substrate temperature, working gas (argon) pressure, and oxygen partial pressure. Structural measurements included x-ray diffraction, scanning electron microscopy, Raman spectroscopy, and infrared spectroscopy. Electrical measurements included resistivity, Hall effect, and optical transmission. Substrate temperature had a strong effect on the crystalline quality of the films as inferred from xrd, infrared, and Raman measurements. Working gas pressure and oxygen partial pressure had a much weaker effect on the crystal structure. Good opto-electronic properties were not always correlated with good crystal quality. In particular good electronic quality films could be produced under deposition conditions that resulted in poor crystal quality. A simple model is presented that relates the crystal quality to the electronic properties accounting for dopant activation, grain boundary scattering, and ionized impurity scattering.

Cadmium telluride (CdTe) is a leading thin film photovoltaic (PV) material due to its near ideal band gap of 1.45 eV, its high optical absorption coefficient and availability of a various device fabrication methods. The status the thin film CdTe solar cell is more than 16.5% efficiency for devices on conducting glass substrates and 7.8% efficiency for devices on flexible metallic substrates. Thin stainless steel (SS) foils are used as the substrate for the development of CdTe solar cells because of its material properties, high temperature stability, commercial availability and cost. A potential problem with the use of a stainless steel foil as the substrate is the diffusion of iron (Fe), chromium (Cr) and other elemental impurities into the layers of the solar cell device structure during high temperature processing. A diffusion barrier limiting the out diffusion of these substrate elements is being investigated in this study. Silicon nitride (Si₃N₄) films deposited on SS foils are being investigated as the transparent barrier layer, to reduce or inhibit the diffusion of substrate impurities into the solar cell. Si₃N₄ coefficient of thermal expansion (CTE) of 4.5x10^-6/°K is close to both the back contact layer Molybdenum, with a CTE of 5.1x10^-6/°K and the absorber CdTe, with a CTE of 5.9x10^-6/°K, minimizing thermal expansion mismatch in the device. It has already been shown by others, that substrate impurities like Fe and Cr in the cell’s absorber can lead to reduced cell efficiencies. In this study, the effect of the Si₃N₄ barrier layer is being evaluated for its effect on cell efficiency and overall device performance. The optimum Si₃N₄ barrier thickness is also being determined. Currently thin film CdTe cells are being fabricated with and without a Si₃N₄ barrier layer. Preliminary results show an improvement in the V_OC of cells fabricated with a 0.1 µm thick Si₃N₄ barrier layer. The thin film CdTe solar cells have been characterized by XRD, SEM, Secondary Ion Mass Spectrometry (SIMS) depth profiles, current-voltage (I-V) characteristics and spectral response.

a-Si_1-XC_X:H alloys are of interest in detectors and white light emitting devices, but their properties under thermal anneal have not been reported so far. This paper considers the processes of charge transfer and trapping in a-Si_1-XC_X:H films deposited on crystalline p-type Si wafers and annealed in vacuum (10^-6 Torr) over temperature range of 300 to 850^oC. The a-Si_1-XC_X:H films were deposited by reactive magnetron sputtering using of the Ar /CH₄ as working gases. An Au/Ti multilayer was used as a contact to the a-Si_1-XC_X:H film and Al as back contact to the Si wafer. Raman scattering spectroscopy, photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) measurements were carried out at room temperature. Current-voltage (I-V) and capacitance-voltage characteristics were measured in the temperature range 100 - 350K.

Evaluation of the I-V characteristics of the initial structure and the structures annealed at 450 ^oC and 650^oC demonstrates that maximum coefficient of rectification is observed for the heterostructure annealed at 450^oC and equals 4x10² for ±5V. For this material the maximum optical band gap and minimum paramagnetic defect concentration are observed. The dielectric constant is found to be 6.5. After 650^oC vacuum annealing the forward and reverse currents are higher than those of both the initial and 450^oC- annealed structures, and are associated with amorphous carbon cluster formation observed by Raman scattering after such thermal annealing. Temperature dependence of forward current of the initial structure demonstrates that variable-range hopping (VRH) conductivity at the Fermi level is dominant up to 1V. The density of states at the Fermi level is estimated at ~8x10¹⁹ cm^-3eV^-1, that is on the order of the concentration of Si and C dangling bonds, determined by EPR technique. Increase of the forward voltage from 0.1V to 1.0V results in increase of average hopping distance from 4.2 nm to 7 nm. Annealing at 450^oC results in change of current transport mechanism: now the forward current can be described by Pool-Frenkel emission from levels with energy ~ 0.11 eV. Annealing at 650^oC considerably reduces the temperature dependence of current, testifying to the emergence of tunneling processes for charge movement. The process of VRH conductivity through a large density of state at Fermi level again appears, with an estimated density of the states around 5x10²⁰ cm^-3eV^-1. Increase of applied voltage beyond |1V| results in a decrease in current with increase of temperature. It is surmised that the observed phenomenon is associated with charge trapping in local regions separated from main matrix by high potential barriers.

Sn-doped In₂O₃ (ITO) film has been used as a transparent electrode for various applications including organic light-emitting diodes (OLEDs) since it combines good conductivity and transparency in the visible region. The work function control of ITO film plays an important role in device parameters such as operation voltage or lifetime for OLED. The work function of ITO film has been controlled by surface treatments by plasma or UV-ozone treatment. However in such methods the work function of ITO film is unstable and can change over time. On the other hand, it is expected that the work function could be controlled by a variation in carrier density. In this study, we investigate how the work function depends on the carrier density of ITO films. The ITO films with various carrier densities were deposited by dc magnetron sputtering on glass substrates heated at 300 ^oC and 400 ^oC using high-density ceramic ITO targets with various SnO₂ concentrations. Total gas pressure and dc power were maintained at 1.0 Pa and 50 W, respectively, for all the depositions. The film thickness of all the ITO films was adjusted as about 200 nm. Carrier density was controlled from 3.06 ×10¹⁹‐5.72×10²⁰ cm^-3 or 4.42 ×10¹⁹‐1.08×10²¹ cm^-3 for the films deposited on the substrates heated at 300 ^oC or 400 ^oC, respectively, by using the ceramic ITO targets with the different SnO₂ concentrations from 0 to 10 wt. %. The increase in the carrier density should be caused by the increase in the substitutional Sn⁴⁺ at In³⁺ sites of In₂O₃. Optical band gap of the films increased with the increasing SnO₂ concentration of the target, where work function decreased. This must be explained quantitatively in terms of the shift of Fermi level with varying carrier density within a parabolic conduction band. The optical band gap or the work function of the films showed clearly positive or negative relationships to the two-thirds power of carrier density, respectively. Furthermore, a detailed analysis was performed using hard X-ray photoemission spectroscopy (HX-PES) in order to investigate the electronic state between the Fermi level and the valence band of ITO films deposited on the substrates heated at 400 ^oC. As a result, the density of state near the Fermi level was found to vary systematically with the carrier density. The synchrotron radiation experiments were performed at the BL47XU in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No.2009A1586).

Recently, the studies on metal-organic interfaces are concentrated in the efficient charge carrier-injection between electrodes and an adjacent organic layer adjoining to electrodes. The efficient carrier-injection properties are very important for improving a luminace and luminous efficiency in the field of the information display such as the organic light-emitting diodes, organic solar cells, organic thin film transistor, and organic sensors, and organic smart window devices. Here, we reported on the new hole-only contact system of 2-TNATA / MoO_x. The organic light-emitting diode with a glass / ITO (85 nm) / MoO_x (5 nm) / 2-TNATA (30 nm) / NPB (18 nm) / Alq₃ (52 nm) / LiF (1 nm)/Al (100 nm) structure showed higher luminous efficiency as two times than the device of the same structure with MoO_x of 0 nm-thick. The improvement of the luminous efficiency by inserting a MoO_x layer between tin-doped indium oxide (ITO) and 2-TNATA is attributed to the lowing of the barrier height in a hole injection (Φ_B^h) as well as the raising the band banding by pinning of Fermi level in the interfaces between two layers. The mechanism for a hole-injecting efficiency from anode to a MoO_x / 2-TNATA layer was proved by analyzing an ultraviolet photoemission spectroscopy (UPS) spectra. Φ_B^h in the 2-TNATA / MoO_x interface with MoO_x of 20 nm-thick was decreased about 2.0 eV, when compared to Φ_B^h of the pure layer with only 2-TNATA.

Keywords: organic light-emitting diode, electronic structure, interface, MoOx, 2-TNATA

In this paper, we report the studies on the hetero-epitaxial growth of wurtzite indium nitride ( InN ) thin films on oxide buffer layer by plasma-assisted chemical beam epitaxy (CBE) system with different III/V ratios. Oxide buffer layer was pre-sputtered using RF sputtering technique before InN deposition. The structural and optical properties of InN films samples were investigated by x-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and temperature-dependence photoluminescence (PL) measurements. The near-infrared emission peak values of samples were between 0.74 and 0.78 eV, which are higher than those pre-reported values explained by the Moss-Burstein effect. While increasing the III/V ratio, the emission PL peak red-shifted. In addition, the temperature-dependence PL spectra exhibit blue-shifted as the measurement temperature increased. We suggest that the blue shift in PL spectra with temperature may result from the variation in concentration of InN films.

Organic light-emitting diodes (OLEDs) are promising for future lighting and display applications. It has been reported, however, that the electroluminescence properties are degraded by self-heating during operation [1,2]. In order to determine the heat propagation mechanism in OLEDs, it is important to measure the thermophysical properties precisely for the components of OLEDs, such as Tris-(8-hydroxyquinoline) aluminum (Alq₃) and N,N'-Di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD) films. Alq₃ and α-NPD are used as the electron-transport/emitting materials and the hole-transport material, respectively. In this study, thermal diffusivity of both the films was characterized quantitatively by ‘rear heating / front detection (RF) type’ nanosecond thermoreflectance systems [3] (NanoTR, PicoTherm), which can directly observe the heat propagation through the film thickness. Alq₃ and α-NPD films sandwiched between aluminum films (Al/Alq₃/Al, Al/α-NPD/Al) were prepared on alkali-free glass substrates by means of vacuum evaporation . The nominal thicknesses of Al, Alq₃ and α-NPD layer were respectively 100 nm, 50-200 nm and 100 nm. The thermal diffusivity of Alq₃ films was found to be 1.4-1.6×10^-7 m²/s, which is about 1.5 times higher than that of Alq₃ powder [4]. Furthermore, the thermal diffusivity of α-NPD films is 1.2×10^-7 m²/s. We also estimated the mean free path of phonons, l_ph, in terms of phonon propagation in Alq₃ films using the thermal conductivity calculated from the thermal diffusivity, heat capacity per unit volume, and the average phonon velocity calculated from Young’s modulus and the density [5]. As a result, l_ph was approximately 0.49 nm, which is smaller than molecular size and intermolecular distance for Alq₃, but almost twice the Al-N bond length [6,7].

<Acknowledgment>

This work was supported by New Energy and Industrial Technology Development Organization (NEDO) as a project of "Development of High-efficiency Lighting Based on the Organic Light-emitting Mechanism".

[1] T. Sugiyama, H. Tsuji, Y. Furukawa, Chem.Phys.Lett. 453 238 (2008).

[2] G. Vamvounis, H. Aziz, N. Hu, Z. Popovic, Synthetic Metals 143 69 (2004).

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Nano-, microcrystalline hydrogenated silicon(nc-, μc-Si:H) attracts much interest as a promising material for thin film solar cells with high performance and good stability compared to amorphous silicon thin film. The deposition of nc-, μc-Si:H has been carried out by using various methods such as hot-wire chemical-vapor deposition (HWCVD), photo-chemical vapor deposition (P-CVD), plasma-enhanced chemical-vapor deposition (PECVD), etc.

During the deposition of nano- micro- crystalline hydrogenated silicon, the defects are generated and the defects in the nanocrystalline silicon thin films can degrade the efficiency of the solar cell. Defects located at deep-gap or tail states in the disordered silicon films are of great importance for the electronic quality of these materials and these states will influence the performance of the solar cells.

In this study, we have investigated the defects in the nancrystalline silicon thin film as a function of crystallization of the nanocrystalline silicon thin film. The nanocrystalline silicon thin film was deposited by using an internal-type inductively coupled plasma system. Electron spin resonance (ESR) is a useful tool for the investigation of defects in amorphous, nano-, and micro- crystalline hydrogenated silcon thin film. The nanocrystalline silicon thin films were deposited on corning 1737 glass. Raman scattering spectroscopy, high resolution transmission electron microscopy, (HRTEM) and electron spin resonance(ESR) were used to evaluate film crystallinity, structural image, and defects in the film, respectively.

Metallic nanophotonic structures demonstrate surfaced enhanced phenomena, thus find their application in device photonics. To facilitate the improvement in the already successful panoply of optical biosensors in general and in the field of water quality in particular, nano-photonic structures such as sculptured thin films (STF) can be used. The existence of localized surface plasmon resonance (LSPR) was observed within metallic STFs. The glancing angle deposition technique (GLAD) by ion beam sputtering and electron beam evaporation was employed to sculpt thin films as a platform for surface enhanced fluorescence (SEF). The self shadowing mechanism is responsible for the growth of non-closed films which consist of needles grown in the direction of the incoming flux of the particles. This thin film deposition method, coupled with an appropriate substrate rotation scheme, enabled to deposit nanorods with less than 30° and greater than 80° inclination with respect to the substrate surface. A multitude of structures were prepared by depositing materials like Ag, Au and Si with GLAD on different substrates such as fused silica, Si(100), Si(100) coated with 15 nm Ti, and on nanosphere lithography pre-patterned substrates that consist of Au and Al nanodots in hexagonal arrangement. The reference (compact) thin films of each material were prepared with the vapor incidence parallel to the substrate normal.

With the integration of a fluorescence microscope with a spectrometer, the green Hg line at 546 nm was used for excitation in most of the SEF experiments and the emission was detected using the red filter at 590 nm. STFs spin coated with a Rhodamine 123 layer of thickness (30-50) nm were observed to show enhancement factors up to few tens. A higher degree of surface enhancement was observed with Ag nanorod STFs inserted in an aqueous solution of E. coli in comparison to corresponding dense Ag reference film.

Lithography at the sub-22 nm length scale will require resist films under 50nm thick with a high degree of homogeneity. Current resists that are in use for ultraviolet lithography may not be suitable for the projected transition to extreme ultraviolet (EUV) wavelengths, leading to active study of alternative materials solutions. One method for gaining sub-nanometer control over the thickness and composition of photoresist film is molecular layer deposition (MLD), which utilizes a series of self-limiting reactions of organic molecules. In this study, a variety of nanoscale organic films were deposited by MLD via urea coupling chemistry, which occurs by reaction of isocyanates and amines. Films were deposited on substrates that were first prepared by vapor deposition of 3-aminopropyltriethoxysilane on hydroxylated SiO₂ surfaces to yield an amine-terminated surface, as confirmed by ellipsometry and XPS. Following amine termination, the diisocyanate and diamine precursors were dosed in a binary cycle, and this cycling was repeated to yield the desired thickness of organic film. Ellipsometry indicates a linear growth rate of 4.5 Å/cycle for the standard coupling of phenylene diisocyanate (PDIC) and ethylenediamine (ED). The urea coupling moiety is confirmed by infrared spectroscopy, and films are shown to have stoichiometric composition by XPS. Temperature dependent measurements show that the films have good thermal stability. To fabricate EUV resists, we have explored a variety of backbones contained within the amine and isocyanate linking groups to tune the functions of the organic films. By changing the backbone of the MLD precursors, we have incorporated ketal-based acid-labile groups into the film and have shown that after incorporation of photoacid generator (PAG), UV exposure, post-exposure bake and development, the films are cleaved, leading to potential use as photoresists. Results of applying the nanoscale oligourea films for advanced photoresist application will be presented.

In situ monitoring of atomic layer deposition (ALD) could potentially make process optimization faster and more cost-efficient. Additionally, it permits computational models for chemistry and fluid dynamics to be tested and refined; these validated models, in turn, could also be useful tools for process development and equipment design. We have been developing real-time diagnostics for gas-phase concentrations using a number of methods including mass spectroscopy, distributed-feedback diode laser absorption spectroscopy, and quantum cascade laser absorption spectroscopy. This work focuses on our work with time-resolved Fourier transform infrared (FTIR) spectroscopy, which we use to monitor gas-phase species during ALD of hafnium oxide from tetrakis(ethylmethylamino) hafnium and water. Results are compared to other measurement techniques applied to the same warm-walled, single-wafer reactor. Additionally, our efforts to model the system using computational fluid dynamics and a detailed kinetic reaction mechanism are discussed.

Doped ceria (where the dopant is a trivalent cation such as Sm or Gd) is an attractive electrolyte material for solid oxide fuel cells (SOFCs) owing to its high ionic conductivity at intermediate temperatures (IT). It is also a good anode material due to its mixed ionic-electronic conduction under the reducing conditions that exist at a fuel cell anode. For high power density, the electrolyte in a fuel cell must be fabricated in thin film form in order to lower the area specific resistance. Accordingly, many studies have been directed towards thin-film preparation of the conventional SOFC electrolyte material, yttria stabilized zirconia. In the present work, samarium doped ceria (SDC) thin films are grown via metal-organic chemical vapor deposition (MOCVD) as a first step towards high power density SDC based fuel cells.

A vertical cold-wall MOCVD reactor was built in-house. This cold-wall reactor has a showerhead which gives impinging multi-jet flow of precursor vapor onto a substrate of choice. Metal organic precursors, Ce(tmhd)₄ for Ce and Sm(tmhd)₃ for Sm, were utilized as the cation source compounds. They are commercially available as fine powders. These were used in the form of fine solid powder coated on steel balls by mechanical stirring. This step is expected to provide uniform mixing of the two precursors with sufficient, stable surface area for evaporation. During a deposition run, minimum surface area change for the solid precursors is desired. The compound mixture was placed in a single evaporation vessel for the simplicity of the system, and the evaporation temperature was controlled.

Precise and reproducible control of composition is not trivial for solid solutions such as samarium doped ceria. An UV optical cell is located between the precursor evaporator and the deposition chamber. Optical absorption of the incoming precursor vapors was monitored in-situ. Even though it is impossible to separate peaks from the two metal-organics due to severe peak overlap, gas phase UV absorption still provide valuable information on evaporation behavior. In-line gas phase reaction forms oxide powders before precursors reach the substrate, and therefore should be suppressed. UV absorption can detect this unwanted reaction. The influence of deposition conditions such as substrate temperature, evaporation temperature, and precursor mixing ratio, on the the samarium content in the oxide thin films is explored. Deposition is carried out on single crystalline oxide wafers, and nickel-SDC cermet pellets. The resulting films were characterized by scanning electron microscopy (SEM), Raman spectroscopy, X-ray diffraction and energy dispersive X-ray spectroscopy (EDS).

Photovoltaics are in a verge of expansion challenged with the grid parity requirement.

Three generations of solar cell technologies are racing for the better $$/Wp numbers.

c-Si, mc-Si, micromorph, Thin film (CdTe, CIGS), Organic – all use thin film processing in order to create absorber, emitter, light trap-assisting layer, back side field -passivating layer, barriers, metallization, etc.

In order to achieve grid parity ($$/Wp) PV-technology strives to increase Power Conversion Efficiency (PCE) coupled with reduced Cost of Ownership (CoO).

Former is done by minimizing losses -optical and electrical and implies advanced interface engineering.

Later entails low cost material and technology utilization that places verdict on industry trend toward innovative technologies.

Wet chemistry based thin film deposition routing is proposed to replace high vacuum ones for several critical layers, such as ARC, TCO, BSF and surface modification.

This low cost method features:

High deposition rate

High material utilization

Simple equipment capable of very large substrate handling.

Ideal for roll-to-roll operation

Full conformity with complex shape surfaces

Highly uniform films of various compositions with the thickness range 6-600 nm have been deposited using dip coating, slot die and microgravure coating, spray coating.

With the proper surface activation film forms covalent bonds with the substrate and has no interfacial imperfectness.

When used for solar cell fabrication – can add up to couple of percent to PCE compare to traditionally processed cells.

Precursor for the specific layer can be purchased or mixed in-house depends on formulation.

Only a limited amount of semiconductor materials have been deployed on a wide-spread basis for terrestrial solar energy production. Single crystalline silicon has held the majority of the market share, with other technologies emerging over the years with a smaller presence (e.g. a-Si:H, CdTe, CIGS). Some of the materials used in many of these technologies possess clear long-term disadvantages with regards to economics, availability, and environmental consequences. A recent study of 23 potential semiconductors ranks the potential of each in regards to their annual electricity production potential and their raw material cost [1]. Detailed experimental research on the photovoltaic potential of many of these individual compounds is very sparse or non-existent.

In this experimental study, seven of the best candidates were selected for further investigation, including FeS₂, Zn₃P₂, PbS, CuO, CuO₂, NiS, and ZnSe. Transmission, reflection, and absorption measurements were performed on these binary compounds. Solar cells were also fabricated at the Florida Solar Energy Center to measure photovoltaic properties, including conversion efficiency under Standard Test Conditions, quantum efficiency and spectral response. The measured optical and electrical properties of these semiconductor materials provides a better understanding of the potential of each for wide-spread deployment. Future work will be focused toward optimizing device design and fabrication processes to maximize the energy conversion of the best potential compounds.

1. C. Wadia, A.P. Alivisatos, D.K. Kammen, Environmental Science and Technology, 2009, 43 (6), 2072-2077

Pt_1-xIr_x films with x = 22.76-63.25 are fabricated on (100) Si substrates at 400 ℃ by magnetron sputtering deposition. Effects of Ir content on the microstructure, morphology and hardness of PtIr films are investigated by field emission scanning electron microscopy, X-ray diffraction, atomic force microscopy and nanoindentation system. The columnar structures are observed by field emission scanning electron microscopy. X-ray diffraction analysis revealed that PtIr films had preferred orientation along Pt(111) as Ir content is below 50.84 at.%. When the Ir content is more than 50.84 at.%, the PtIr film had another preferred orientation, Ir(111).The surface morphology is analyzed by atomic force microscopy. Roughness of PtIr films is decreased with increased Ir content. The hardness of all PtIr films is under 20 GPa. It is found the maximum hardness of PtIr films is about 14.9 GPa as Ir content is 57.9 at.%.