The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Amorphous Oxides II
Chair: Rodrigio Martins, Universidade Nova de Lisboa
Strong Blue-green Photoluminescence from a-SiNxOy Films with Internal Quantum Efficiency Exceeding 62%
State Key Laboratory of Solid State Microstructures and School of Electron Science and Engineering, Nanjing University, Nanjing 210093, P. R. China
To achieve the high quantum efficiency of the photoluminescence (PL) and electroluminescence (EL) is the main objective of the research work in the field of silicon based optoelectronic devices. In this paper we report the high PL quantum efficiency from the luminescent N-Si-O bonding configuration in a-SiNxOy films. The samples were prepared by PECVD technique with SiH4 and NH3 gas mixture and subsequently oxidized in situ by oxygen plasma treatment. The N-Si-O bonding configuration was inspected by X-ray photoelectron spectroscopy (XPS). The binding energy of Si 2p peak is around 102.2 eV for our samples which are greater than 101.9 eV for Si3N4 and less than 103.4 eV for SiO2. From the results of optical absorption, PL and PLE spectra, we can estimate that the luminescent N-Si-O configuration states located in the gap at 0.6 eV below the conduction band. A room temperature strong blue PL peaked at around 488 nm has been observed.
The temperature dependent PL (TD-PL) measurements are widely used to measure the radiative efficiency of the luminescent semiconductor materials and LED, which is based on assuming 100% internal quantum efficiency (IQE) at low temperature . We measured the TD-PL spectra in the temperature range of 10–300 K and took the ratio of the integrated PL intensity at 300K to that at 10K, to get the experimental data of IQE. The room temperature PL IQE of 62% peaked at 488 nm from our a-SiNxOy sample has been achieved. This result of room temperature PL IQE of Si based luminescent materials is higher than that of previous report highest PL quantum yield of 60% from silicon quantum dots by wet-chemical surface passivation with organic ligands .
On the other hand, the characteristic of time resolved PL (TR-PL) has also been investigated. The integrated PL dynamics was measured at 487 nm in the microsecond and nanosecond regimes. In the PL time decay spectra there are two distinct PL decay time which are a "fast" radiative lifetime of 4.56 ns and a "slow" radiative lifetime of 25.49±1.02 μs. Based on the luminescent N-Si-O state located in the band gap and the very short PL lifetime, we propose a three band level model to explain the PL mechanism and high IQE in our a-SiNxOy films. The fast nanosecond PL decay component is associated with the nonradiative excited electrons relaxing to the luminescent N-Si-O bonding states present in the band gap which the longer microsecond decay results from the recombination, both radiative and nonradiative, of the relaxed electrons. The resulting very short radiative lifetime and three band level structures predetermine this kind of a-SiNxOy films for further investigation related to stimulated emission.
 W. Z. Liu, H. Y. Xu, et.al, Appl. Phys. Lett., 101, 142101 (2012)
 D. Jurbergs, E. Rogojina, et.al, Appl. Phys. Lett., 88, 233116 (2006)
Keywords: a-SiNO film, photoluminescence, internal quantum efficiency, PL lifetime, three-level model
Control of Growth Process for Obtaining High Quality a-SiO:H
1. Graduate School of Engineering Science, Osaka University, Toyonaka Osaka, 560-8531, Japan
2. JST-CREST, Toyonaka Osaka, 560-8531, Japan
Control of optoelectronic properties in wide-gap semiconductors is an important issue for improving photovoltaic properties in thin film Si-based solar cells. In this study, we have investigated the film-growth processes to control the optoelectronic properties in hydrogenated amorphous silicon-oxygen alloys (a-SiO:H) prepared by plasma-enhanced chemical-vapor deposition (PECVD) method.
a-SiO:H films were prepared on glass substrate by the capacitively-coupled RF (13.56 MHz)-PECVD system using CO2/SiH4 gas mixture. Total gas pressure and gas-flow-rate ratio [CO2/(SiH4+CO2)] were fixed. Total gas-flow rate, input power density and substrate temperature were varied from 5 to 18 sccm, 1 to 5 W, and 120 to 200°C, respectively. As an in-situ plasma-diagnostic technique, optical emission spectroscopy (OES) was used, in which the emission intensities from emissive species (CO* [519.8 nm], O2* [337.0 nm], and SiH* [414.2 nm]) were monitored. X-ray photoelectron spectroscopy (XPS) was used to determine the elemental composition in the resulting films.
In pure CO2 plasma, it is considered that the decomposition reaction "CO2 + e → CO + O" is the majority reaction due to its lowest dissociation energy. Furthermore, CO* and O2* emission intensities show linear electron-density dependence, indicating that CO* and O2* emissions take place via one-electron-impact process in CO2 plasma. Therefore, formation process of a variety of reactive species such as CO, O and O2, exhibiting lower formation energy than the emissive species, must occur via one-electron-impact process to CO2 molecules in the plasma.
Carbon signal is weaker than the detection limit of XPS in the resulting films prepared from a wide range of CO2/(SiH4+CO2) plasmas, suggesting that CO reaching the film-growing surface shows quite low sticking probability. In addition, linearity of SiH* emission intensity vs. electron density is lost when SiH4 is mixed with CO2, indicating that SiH4 molecules is strongly depleted by the reaction of SiH4 with O atoms forming OH. Therefore, main film-growth precursors of a-SiO:H are considered to be SiH3, OH, and O.
Understandings of a-SiO:H-film-growth process derived from our studies is as follows; SiH3 reaching the film-growing surface abstracts surface-covering-bonded hydrogen forming Si-dangling bond (defect and growth site). OH also abstracts the surface hydrogen forming defect. It should be noted here that the short lifetime Si-related species (SLS; Si and SiH) exhibits the insertion reaction into the surface Si-H bond also forming defect when it contributes to film growth. On the other hand, Si growth occurs by the surface-defect-annihilation reaction done by surface-diffusing SiH3. Surface-reaction-rate constant of SLS-insertion reaction is considered to be much higher than that of other surface-defect-formation reactions. Since the gas-phase SLS-scavenging reaction only with SiH4 is hard to occur under the SiH4-depletion conditions, dangling-bond-defect density in the resulting a-SiO:H shows higher value by one or two orders of magnitude as compared to that in a-Si:H. On the basis of microscopic understandings for the defect-formation process during a-SiO:H-film growth, we have succeeded to prepare high quality a-SiO:H showing dangling-bond-defect density as low as 3x1015 cm–3. Namely total gas-flow-rate was increased and power density was decreased to avoid the SiH4 depletion for reducing the defect-generation rate by surface-arriving SLS on the film-growing surface. At the same time, substrate temperature was increased to enhance the surface-diffusion coefficient of SiH3 for promoting the defect-annihilation rate on the film-growing surface.
Keywords: amorphous Si-O alloys, process control, low defect density
Fabrication of SiOx Thin Films by Pulsed Laser Deposition
Department of Physics, Indian Institute of Technology Guwahati, 39 India
Bulk crystalline silicon is inefficienct to emit light due to its indirect band gap. The nanostructured silicon shows intense visible and near infrared photoluminescence (PL). Wide range of tenability over the refractive index of SiOx may make it possible to fabricate graded refractive index based photonics device. This could be applied in the fabrication of wideband antireflection coatings and wideband reflection mirrors.
Nanostructured Si-rich silicon oxide (SiOx, 0 < x < 2) films have been fabricated by pulsed-laser deposition (PLD) of Si in oxygen ambient at different pressure ranging from 5x10–5 to 1 mbar in the present report. Films were deposited using a pulsed Q-switched Nd:YAG laser ( 532 nm, 10 ns duration, 10 Hz) at a laser fluence of 2.5 J/cm2 and substrate temperature of 400°C. These thin films were characterized by photoluminescence and Raman spectroscopy. The surface morphology and crystal structural were recorded by scanning electron microscope and Laser Raman Raman spectrometre. The effect of O2 pressure on the optical properties of nano structured Si:SiOx thin films at room temperature (RT) were also studied by the UV-Vis-NIR transmission and PL spectroscopy.
The as-deposited SiOx nanostructured films show micron-sized particles on a uniform background. The Raman spectra of the SiOx films showed broad peaks in wave number range 100–650 cm–1.The spectra was fitted to multiple lorentzian lines which showed broad peaks at 145 cm–1, 332 cm–1, 487 cm–1, and 430 cm–1 corresponding to transverse acoustic (TA), longitudinal acoustic (LA), transverse optic (TO) phonon modes of α-Si and 6 membered ring in Silica(Si2O4) network respectively. This confirmed that the thin films were basically composed of α-Si embedded in SiO2 matrix. The micron sized particles showed asymmetric broad band PL emission with peak around 709 nm (1.75 eV) ranging from 600–800 nm. But no PL is observed from the background films. This red PL could be due to α-Si nanostructures, O2 related defects or both as the presence of both were confirmed by Raman spectra.
The UV-Vis-NIR transmission spectra of the SiOx films were recorded from wavelength range of 200 nm to 3000 nm. The absorption coefficients and thickness of films were measured from interference pattern of the transmission spectra using envelop method. Band gaps calculated using Tauc plot shows systematic blue shift of band gap energy from 1.5–2.8 eV approximately with increasing O2 pressure from. The refractive index (n) and extinction coefficients (k) were also calculated for UV-Vis-NIR region. Refractive index of the films were found to vary from 4.54–1.68 (within λ ~1000 nm to 3000 nm) with the increase in O2 pressure. Hence the ambient O2 pressure, which affects the O2 constituent in the films, has strong influence in the optical constants and band structures in the films.
Keywords: Si :SiO2 matrix, pulsed laser deposition, optical constants, envelop method, Nd-YAG laser
Quantum Confinement Effects in Amorphous In-Ga-Zn-O
1. Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatuda, Midori-ku, Yokohama 226-8503, JAPAN
2. Material Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatuda, Midori-ku, Yokohama 226-8503, JAPAN
3. Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatuda, Midori-ku, Yokohama 226-8503, JAPAN
We studied quantum confinement effects in amorphous In-Ga-Zn-O (a-IGZO) thin films. Superlattice structures were fabricated with a-IGZO and amorphous Ga2O3 (a-Ga2O3) films, in which the a-IGZO film with a Tauc gap of 3.1 eV forms a well layer, and the a-Ga2O3 film with 4.3 eV forms a barrier layer. All the layers were deposited by conventional radio-frequency magnetron sputtering at room temperature. The Tauc gap of the superlattice increases with decreasing the well thickness at ≤5 nm. The well thickness dependence is quantitatively explained by the Krönig-Penny model employing a conduction band offset of 1.2 eV between the a-IGZO and the a-Ga2O3 layers, and the effective masses of 0.35m0 for the a-IGZO and 0.5m0 for the a-Ga2O3, where m0 is the electron rest mass. This result demonstrates the quantum confinement effect and corresponding bandgap shift in the a-IGZO well layer. The phase relaxation length is estimated to be larger than 3.5 nm.
Additionally, a-IGZO TFTs with a single a-IGZO well were also fabricated. The single a-Ga2O3/a-IGZO/a-Ga2O3 well was formed on the gate insulator of the TFT, and an additional 20 nm thick a-IGZO film was deposited to cover the a-IGZO single well. The TFTs show different current-voltage (I-V) characteristics from those of the ordinary a-IGZO TFT (i.e. a 20 nm thick single-layer a-IGZO channel is employed). When the well thickness was 5 nm and below, plateaus were observed in the field effect mobility as a function of gate voltage for the single-well TFTs. The plateau starting voltage increases with decreasing the well thickness, and the plateau mobility depends on the well and barrier thicknesses The device simulation considering the bandgap shifts of the a-IGZO well originating from the above quantum confinement effects reproduced the well thickness dependence of the plateau staring voltage, and the mobility plateau started at the gate voltage where the electron accumulation started to extend the a-IGZO well. This result clarified that the I-V characteristics are mainly determined by the electronic structure of the quantized a-IGZO well, when the gate voltage is above the starting voltage.
Keywords: a-IGZO, quantum confinement, superlattice, thin-film transistor, field effect mobility
Silicon Oxide Interlayers in Hot Wire Chemical Vapor Deposition of a Silicon Nitride/Polymer Thin Film Moisture Barrier
1. Nanophotonics - Physics of Devices, Debye Institute for Nanomaterials Science, Utrecht University, High Tech Campus, Building 5, 5656 AE Einhoven, The Netherlands
2. Energy Research Centre of the Netherlands - Solliance, High Tech Campus 5, 5656 AE Eindhoven, The Netherlands
3. Eindhoven University of Technology (TU/e), Department of Applied Physics, Plasma & Materials Processing, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Sensitive electronic devices, especially those using flexible substrates, need a thin film water and oxygen barrier layer, to prevent them from degrading too quickly.
We deposited a silicon nitride (SiNx)/polymer hybrid multilayer moisture barrier in a hot wire chemical vapor deposition (HWCVD) process, entirely below 100°C. The polymer, in our case poly(glycidyl methacrylate) (PGMA), was deposited by initiated chemical vapour deposition (iCVD) and the SiNx in a dedicated HWCVD reactor. We were able to deposit a simple pinhole-free stacked thin layer structure, consisting of only two low-temperature SiNx layers with a defect-decoupling PGMA interlayer. It exhibits ultrahigh barrier properties: a water vapor transmission rate (WVTR) as low as 5x10–6 g/m2/day at a temperature of 60°C and a relative humidity of 90% . This is low enough for any sensitive electronic devices.
High interfacial adhesion between the sublayers was shown by focussed ion beam (FIB) and scanning electron microscopy (SEM). Line profile investigation of our barrier structures by cross sectional scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectrometer (EDX) reveals that, upon HWCVD of SiNx on top of our polymer layer, an initial layer of silicon oxide (SiOx)-like material is formed. The presence of this material is confirmed by X-ray photoelectron spectroscopy (XPS) measurements. We hypothesize that oxygen atoms are released from the epoxide groups contained in PGMA, upon heating (to 100°C) and exposure to atomic hydrogen in the HWCVD process. These react with silane to form SiOx. This interlayer can be highly beneficial and is probably one of the reasons for the excellent barrier properties of our multilayer and high interfacial adhesion internally in our multilayer. Layers of SiOx and silicon oxynitride (SiOxNy), because of their slightly higher plasticity, are sometimes incorporated as additional inorganic layers on SiNx layers to cover pinholes in thin film moisture barriers .
In summary, we present an efficient process of depositing a three layer barrier structure in two deposition steps, which could open up new possibilities in the deposition of hybrid multilayers for many applications.
 D.A. Spee, C. H. M. van der Werf, J. K. Rath, and R. E. I. Schropp, Phys. Status Solidi RRL 6 (2012) p. 151
 Y. Ogawa, K. Ohdaira, T. Oyaidu, and H. Matsumura, Thin Solid Films 516 (5) (2008) p. 611
Keywords: thin film gas barrier, flexible electronics, hot wire chemical vapor deposition, initiated chemical vapor deposition