The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Amorphous Oxides III
Chair: Rodrigio Martins, Universidade Nova de Lisboa
Boron Doping in a-SiO:H
1. Graduate School of Engineering Science, Osaka University, Toyonaka Osaka, 560-8531, Japan
2. JST-CREST, Toyonaka Osaka, 560-8531, Japan
Wide gap p-type window layers have been used in thin film Si-based solar cells to transmit sun light into photoactive i layer as much as possible. Hydrogenated amorphous silicon-carbon alloys (a-SiC:H) and hydrogenated amorphous silicon-oxygen alloys (a-SiO:H) have been proposed and used as the window layers. Recently, film-growth process of a-SiO:H from CO2/(CO2+SiH4)-glow-discharge plasma has been investigated in detail. As a consequence, high quality a-SiO:H with low dangling-bond-defect density has been successfully prepared. In this study, boron (B)-doped a-SiO:H has been investigated for checking the applicability of a-SiO:H to wide gap p-type window layer in solar cells.
B-doped p-type a-SiO:H (p-a-SiO:H) films were prepared using the capacitively-coupled RF (13.56MHz) plasma-enhanced chemical-vapor deposition (PECVD) system using B2H6/SiH4/CO2/H2 source-gas mixtures. CO2/(CO2+SiH4) of 0.6 was selected as a starting-gas composition, showing the optical band gap between 2.0 eV and 2.2 eV in the intrinsic a-SiO:H. Doping ratio (B2H6/SiH4) was varied from 0 to 10000 ppm. Substrate temperature, input RF-power density, total gas-flow rate and working pressure were kept constant at 140°C, 0.06 Wcm–2, 10 sccm, and 60 mTorr, respectively. Dark conductivity and its activation energy were measured by the conductivity-measurement system with thermal cryostat, optical gap was estimated from transmittance-wavelength spectrum, and optical absorption-coefficient at longer wavelength region was calculated by the result of constant photocurrent method (CPM). Film composition (Si, O, H) is determined by X-ray photoelectron spectroscopy (XPS) and infrared-absorption spectroscopy (IR). Boron content was measured by secondary ion mass spectroscopy (SIMS).
Optical gap is decreased with increasing B-doping level in a-SiO:H similar to a-Si:H, being due to the catalytic H-removal process with BHx chemical species arriving on the film-growing surface. However, decrease of optical gap with B-doping level is much severer in a-SiO:H as compared to in a-Si:H, being due to the decrease of O content together with the decrease of H content in the case of a-SiO:H. B-incorporation efficiency (B/Si) determined by SIMS in a-SiO:H was almost the same as in a-Si:H. However the dark conductivity in B-doped p-type a-SiO:H shows much lower value by almost two orders of magnitude as compared to that in B-doped p-type a-Si:H in the whole B-doping-level range.
To know the reason why the dark conductivity is low in p-type a-SiO:H, the activation energy of dark conductivity was measured in many as-prepared and thermally annealed samples with the same doping level of 5000 ppm. The dark conductivity in p-type a-SiO:H is always low as compared to a-Si:H showing the same activation energy. Urbach energy of high quality intrinsic a-SiO:H was measured by CPM. It should be noted here that CPM measurement and exact comparison of Urbach energy are possible only when the samples shows similar defect density as low as 1015 cm–3. Urbach energy of a-SiO:H is 65.1 meV, while that of a-Si:H is 44.7 meV for almost the same defect-density samples of 3x1015 cm–3. These experimental results mentioned above indicate that carrier (hole) mobility is quite low in a-SiO:H by almost two orders of magnitude due to the different band structure of a-SiO:H from a-Si:H especially at the valence band tail. Therefore, lower dark conductivity in B-doped p-type a-SiO:H as compared to that in B-doped p-type a-Si:H is owing to its lower carrier (hole) mobility, presumably due to the presence of O2p lone-pair states at the top of valence band in a-SiO:H.
Keywords: p-type amorphous Si-O alloy, carrier transport, Urbach energy
Electronic Structure within The Mobility Gap and Photoinduced Instability of amorphous IGZO
Department of E&EE, Graduate School of Industrial Technology, Nihon University, 1-2-1 Izumicho, Narashino, Chiba 175-8575 Japan
We have investigated the photo-induced instability of amorphous InGaZnO4 (a-IGZO) film and compared with thin film transistors before and after illumination under negative gate biases. The electronic structure of the a-IGZO is evaluated by constant photocurrent method (CPM), Photo Yield Spectroscopy (PYS) and Inverse Photo-Emission Spectroscopy (IPES). The film was deposited onto a quartz substrate from 45 nm to 2000 nm by sputtering method. In order to investigate the change in TFT characteristics by negative bias illumination stress, CPM spectra are measured in the same TFT.
The optical band gap of a-IGZO is around 3.02 eV. The subgap spectra evaluated by CPM can be measured from the valence band edge to near midgap. 2.4 eV and ~1.5 eV absorption levels below the conduction band edge are observed characteristically in every a-IGZO spectrum independently of film thickness. Both the absorption levels increase by light soaking at 400 nm of light for 1 hour in the air. The spectrum reverses by annealing at 350°C for 1 hour in the air. These phenomena are hardly observed in the TFT ID-VG or ID-VD characteristics. On the other hand, in the light soaking under negative gate bias, the 2.4 eV and ~1.5 eV levels increase. The reversible change is also observed in the same annealing conditions. These phenomena are observed correspondence with the TFT characteristics as negative VT shifts. In summary, it is shown that the light induced degradation occurs whole the IGZO film and the degradation is accelerated by negative gate bias, despite that the detail of the degradation mechanism remains to be seen.
Keywords: InGaZnOO4, NBIS, CPM, meta stability, subgap absorption
Fr-A3.3 (invited) 14:40–15:10
Thin Film Uncooled Micro-bolometers Based on Plasma Deposited Materials
1. National Institute for Astrophysics, Optics and Electronics, Puebla, 72840, Mexico
2. Universidad Autonoma de Ciudad Juarez, UACJ, Ciudad Juarez, Chihuahua, Mexico
This presentation summarizes our work on thin film un-cooled micro-bolometers (MBs) made of materials deposited by plasma enhanced chemical vapor deposition (P CVD). The work was performed in National Institute for Astrophysics, Optics and Electronics, Puebla, Mexico in comparison against the data reported in the literature. Micro-bolometers are devices that sense radiation via heat of absorbing layer resulting in changes of electrical properties e.g. resistivity. Therefore temperature coefficient of resistivity (TCR) is a principal parameter that determines performance. Plasma deposited materials: non crystalline semiconductors that provide high TCR values, dielectrics like silicon oxide and silicon nitride that are used for thermo isolation together with surface micromechanical technology pave new ways for thin film uncooled micro-bolometers making these devices promising for 2D imagers both in IR and in THz region. We used a-Si:H(B), Ge:H, GeSi:H, GeSiB:H, and polymorphous p-Ge:H, p-SiGe:H materials with large TCR as thermo sensing film. Fabrication is described for two types of MB configurations (membrane and bridge) with planar and sandwich electrodes. Bridge configuration is attractive because read-out circuitry is not on the frontal surface resulting in more effective use of device frontal area. Plasma deposited silicon nitride films were used as both support layer and optical coating to improve response in the range of 8–12 μm. 2D modeling revealed both linear (commonly observed) and super linear behavior of response to radiation intensity. Voltage responsivity observed ranged RU from 1.2x105 V/W to 7.2 V/W and is approximately the same in both planar and sandwich MBs, while sandwich structure shows current responsivity RI= 14 A/W by about three orders of value higher than that for planar structure.
Key issue for any detector is a signal-to-noise ratio characterized by detectivity. Noise spectra of thermo-sensing film as main contributor to device noise are presented and discussed. We demonstrate that different thermo-sensing materials show significant difference in noise characteristics. Noise in sandwich configurations is by orders of value higher than that in planar structures.
The best parameters observed with Ge-Si:H thermo-sensing film are: voltage sensitivity RU= 7.2 V/W current sensitivity RI = 14 A/W voltage detectivity DU = 8x109 cmHz1/2W–1 and current detectivity DI = 4x109 cmHz1/2W–1. Thin film junctions like Schottky barrier or p-i-n structures fabricated on top of the bridge or on membrane for thermo isolation can be also used as thermo-sensing element. We discuss their characteristics and advantages. Finally we describe some available and potential applications of thin film uncooled micro-bolometers.
Keywords: uncooled micro-bolometers, plasma deposition, silicon-germanium