D.S.H. Chan

National University of Singapore, Singapore, Singapore

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Publications (174)245.78 Total impact

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    ABSTRACT: Experimental results show that the V <sub>OC</sub> of layered heterojunction (HJ) organic photovoltaic (PV) cells behaves with a very weak dependence on the electrodes. However, the V <sub>OC</sub> of bulk HJ PV cells behaves with a strong dependence on the electrodes. In this paper, an explanation for the different behaviors of V <sub>OC</sub> on the electrodes is proposed. It is found that the V <sub>OC</sub> of the two types of PV cells follows the same mechanism and is mainly determined by the light-injected carriers at the donor/acceptor (D/A) interface and the electrodes. However, the distinct device structures make the boundary conditions in layered and bulk HJ PV cells different, which leads to the different dependences of V <sub>OC</sub> on the electrodes. The layered HJ PV cells have geometrically ¿flat¿ D/A and metal/organic (M/O) interfaces (the interface near the electrode), which makes the effective thickness from the D/A interface to the M/O interface large. Thus, there is a low electric field at the M/O interface and, then, a very small barrier lowering. Under this condition, the light-injected carriers at the D/A interface tend to ¿pin¿ the Fermi level of the electrodes. As a result, V <sub>OC</sub> shows only a very weak dependence on the work function of the electrodes. However, the formation of the interpenetrating network in bulk HJ PV cells greatly decreases the D and A domain dimensions and induces the ambipolar carrier distribution in the blend layer. This will cause very large barrier lowering at the M/O interface when there is a high barrier. Under this condition, the light-injected carriers at the D/A interface can no longer ¿pin¿ the electrode Fermi level. Thus, a strong dependence of V <sub>OC</sub> on the electrodes for bulk HJ PV cells is observed.
    IEEE Transactions on Electron Devices 03/2010; · 2.06 Impact Factor
  • Lu Zhang, Wei He, D.S.H. Chan, Byung Jin Cho
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    ABSTRACT: Metal-insulator-metal (MIM) capacitors fabricated with (8%) La-doped HfO<sub>2</sub> single layer as well as HfLaO/ LaAlO<sub>3</sub>/HfLaO multilayer dielectric stack are demonstrated. While the La-doped HfO<sub>2</sub> single layer is crystallized at 420<sup>??</sup>C annealing, HfLaO/LaAlO<sub>3</sub>/HfLaO multilayer dielectric stack remains amorphous. A high dielectric-constant value of 38 can be obtained when 8% La-doped HfO<sub>2</sub> is crystallized into cubiclike structure. However, it is observed that the linearity of MIM capacitor is degraded upon crystallization. The multilayer film has lower average dielectric constant but shows low quadratic voltage linearity of less than 1000 ppm/V<sup>2</sup> up to a capacitance density of 9 fF/??m<sup>2</sup> . It is observed that the HfLaO single-layer MIM is suitable for the applications with requirements of high capacitance density and robust reliability, while the multilayer MIM is suitable for a precision circuit.
    IEEE Electron Device Letters 02/2010; · 2.79 Impact Factor
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    ABSTRACT: Aluminum-doped gadolinium oxides GdAlO x are proposed as a blocking oxide layer in charge-trap-type flash memory cell devices. Greatly improved operation speed and charge retention properties have been demonstrated, compared to conventional Al<sub>2</sub>O<sub>3</sub> blocking layer. The optimization of Al percentage in GdAlO x , as well as charge loss mechanism in the memory cell device, has also been systematically studied.
    IEEE Transactions on Electron Devices 12/2009; · 2.06 Impact Factor
  • Wei He, D.S.H. Chan, Lu Zhang, Byung Jin Cho
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    ABSTRACT: It is demonstrated that HfO<sub>2</sub> films can have much higher dielectric-constant values than the usual reported value of 20-24 by optimized incorporation of lanthanum element and crystallization to cubic structure. When HfO<sub>2</sub> with 8% La is crystallized into cubic structure, the film exhibits the kappa value of ~38 which is the highest among ever reported HfO<sub>2</sub> -based high-kappa dielectrics. The increased kappa value of HfO<sub>2</sub> with 8% La enables the leakage current to be reduced more than one order of magnitude lower, compared to amorphous-phase HfO<sub>2</sub> under the same electric field. The dependence of film thickness and annealing temperature on the cubic crystallization is also reported.
    IEEE Electron Device Letters 07/2009; · 2.79 Impact Factor
  • Lu Zhang, Wei He, Daniel Chan, Byung Jin Cho
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    ABSTRACT: Metal-Insulator-Metal (MIM) capacitors with various thickness as (22 nm, 30 nm, 37 nm and 44 nm) of La (8%) doped HfO2 deposited using atomic layer deposition were fabricated. A high dielectric constant value of 38 can be obtained when 8% La doped HfO2 is crystallized into cubic structure. While amorphous HfLaO demonstrates a quadratic voltage linearity
    ECS Transactions 05/2009; 19(2).
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    ABSTRACT: We demonstrated, for the first time, p-MOSFETs (L<sub>G</sub> ges 40 nm) with SiGe/Si core/shell channel integrated on bulk Si using a CMOS-compatible top-down processes. The Omega-shaped nanowire (NW)-like channels comprised of ~12-nm-thick inner SiGe core and 4-nm-thick outer Si shell. The devices exhibited good subthreshold characteristics (with SS ~128 mV/dec), suggesting successful surface passivation of the SiGe NW body by the outer Si capping layer. Drive currents of ~167 muA/mum is achieved, which is 15% enhancement over the reference Si-channel devices fabricated by the same process. Double g<sub>m</sub> peaks are observed at low drain bias for the core/shell SiGe NW devices, confirming the quantum confinement of holes in the SiGe inner core.
    IEEE Electron Device Letters 05/2009; · 2.79 Impact Factor
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    ABSTRACT: A significant increase in open circuit voltage (VOC) is obtained in the polymer-fullerene bulk heterojunction solar cell by using the e-beam deposited Al cathode. Compared with the device with the thermal evaporated Al cathode, an obvious enhancement of VOC from 596 to 664 mV is obtained, which makes the overall device power conversion efficiency improved by 12.4% (from 3.79% to 4.26%). Electrical characterizations suggest that the energetic particles in the e-beam deposition induce deep interface hole traps in the poly(3-hexylthiophene-2,5-diyl) (P3HT), while leaving the fullerene unaffected. The deep trapped holes near the P3HT/cathode interface can induce the image negative charges in the cathode and thus form “dipoles.” These dipoles lead to the lowering of the Al effective work function and cause the enhancement of VOC.
    Applied Physics Letters 03/2009; 94(10):103305-103305-3. · 3.52 Impact Factor
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    ABSTRACT: This letter demonstrates successful integration of Gate-All-Around (GAA) nanowire (NW) transistors with low-resistivity metallic NW point contacts at source/drain extensions. Ultrascaled GAA silicon NW transistors with gate lengths down to 8 nm have been achieved, exhibiting good performance among the NW FETs reported to date. Compared to the reference devices, the metallic contact NW devices show 580% enhancement in I <sub>ON</sub> from 103 to 705 muA/mum , at a fixed I <sub>OFF</sub> of 10 nA/mum . Nickel silicide resistivity for ultrathin films is also investigated in this letter for the integration of salicided source/drain extensions with the GAA NW process. Experimental results show that 4 nm of deposited Ni is suitable for forming NW contacts with 10-nm diameters, which is thin enough to avoid oversilicidation while meeting the low-resistivity requirements.
    IEEE Electron Device Letters 03/2009; · 2.79 Impact Factor
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    ABSTRACT: A simple and cost-effective single metal gate scheme was successfully demonstrated to form gate-all-around (GAA) nanowire FETs with optimized dual V<sub>T</sub> for low power CMOS applications. FUSI gate-induced stress effects were shown to be of great relevance to device performance. At an I<sub>Off</sub> of 20 pA/mum, superior I<sub>On</sub> of 1180 and 405 muA/mum were obtained for NFETs and PFETs at a V<sub>DD</sub> of 1.2 V.
    Electron Devices Meeting, 2008. IEDM 2008. IEEE International; 01/2009
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    ABSTRACT: In this work, we investigate the effect of energy band profile modulation on carrier backscattering in SiGe nanowire (SGNW) heterojunction p-channel field effect transistors. The energy band profile is modulated by increasing the Ge mole fraction in nanowire channels as compared to source/drain regions using the pattern-dependent Ge condensation technique. The carrier backscattering characteristics of the fabricated heterojunction p-type SGNW transistors, extracted using a temperature-dependent analytical model, exhibited a decrease of 19% in hole backscattering coefficient in comparison to the reference planar devices with uniform Ge concentration. The reduction in backscattering coefficient is attributed to KT/q barrier layer thinning of the source-to-channel barrier for the holes as a result of the modulation in energy band profile caused by variation in Ge concentration.
    Applied Physics Letters 12/2008; 93(25):253105-253105-3. · 3.52 Impact Factor
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    ABSTRACT: Near-infrared photon emission spectra were obtained from the frontside of silicon nMOSFETs and pMOSFETs with a gate length of 0.13 mum and biased into saturation. These spectra were obtained using a high sensitivity in-lens spectroscopic photon emission microscope. Frontside NIR photon emission spectroscopy are performed on 0.13 mum saturated nMOSFETs and pMOSFETs at different gate and drain bias. The nMOSFETs photon emission spectra obtained are significantly different from some previously reported photon emission spectra. The NIR photon emission spectra of the nMOSFETs and pMOSFETs have similar peaks and suggest that the electric field condition in the channels of the nMOSFETs and pMOSFETs are similar.
    Physical and Failure Analysis of Integrated Circuits, 2008. IPFA 2008. 15th International Symposium on the; 08/2008
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    ABSTRACT: A multilayer structure of copper phthalocyanine/poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl- C <sub>61</sub> -butyric acid methyl ester (CuPc/P3HT:PCBM) is used to extend the light absorption spectrum covering almost the entire visible spectrum. To maximize the light absorption, the total number of excitons created in the multilayer structure as a function of layer thickness of both CuPc and P3HT:PCBM is simulated by using the optical transfer matrix formalism. The solar cells with a device structure of ITO/PEDOT:PSS/CuPc/P3HT:PCBM/Al are fabricated with different layers thicknesses. The optimized solar cell with a high short circuit current density of 12.54 mA / cm <sup>2</sup> and power conversion efficiency as high as 4.13% is achieved, owing to the utilization of the second optical interference peak in the multilayer structure for the enhanced light absorption.
    Applied Physics Letters 08/2008; · 3.52 Impact Factor
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    ABSTRACT: A simple method is developed to make an interpenetrating network of poly(3-hexylthiophene-2,5-diyl) (P3HT) and fullerene (C60) by mixing P3HT solution with a thermal initiator 2,2′-azobis(isobutyronitrile) (AIBN). After mild annealing, the release of nitrogen from AIBN increases the roughness of P3HT dramatically. Significant photoluminescence quenching between the roughened donor P3HT and overlaying acceptor C60 is related to the significant increment of donor-acceptor interfacial areas. Based on this interpenetrated network of P3HT/C60, more than threefold increase in the photovoltaic efficiency of devices is achieved compared with bilayer structure. Fill factor is also improved, implying good percolation path in this heterojunction structure.
    Applied Physics Letters 07/2008; 93(4):043304-043304-3. · 3.52 Impact Factor
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    ABSTRACT: Parasitic S/D resistances in extremely scaled GAA nanowire devices can pathologically limit the device drive current performance. We demonstrate for the first time, that S/D extension dopant profile engineering together with successful integration of low resistivity metallic nanowire contacts greatly reduces parasitic resistances. This allows 8 nm gate length GAA nanowire devices in this work to attain record-high drive currents of 3740 muA/mum.
    VLSI Technology, 2008 Symposium on; 07/2008
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    ABSTRACT: A top-down approach of forming SiGe-nanowire (SGNW) MOSFET, with Ge concentration modulated along the source/drain (Si<sub>0.7</sub>Ge<sub>0.3</sub>) to channel (Si<sub>0.3</sub>Ge<sub>0.7</sub>) regions, is presented. Fabricated by utilizing a pattern-size-dependent Ge-condensation technique, the SGNW heterostructure PMOS device exhibits 4.5times enhancement in the drive current and transconductance (G<sub>m</sub>) as compared to the homojunction planar device (Si<sub>0.7</sub>Ge<sub>0.3</sub>). This large enhancement can be attributed to several factors including Omega-gated nanowire structure, enhanced hole injection efficiency (due to valence band offset), and improved hole mobility (due to compressive strain and Ge enrichment in the nanowire channel).
    IEEE Electron Device Letters 07/2008; · 2.79 Impact Factor
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    ABSTRACT: We proposed and demonstrated a simple tandem structure of organic photovoltaic (PV) cell for efficient light harvesting. In this device structure, a soluble fullerene derivative of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is employed simultaneously to form a bilayer heterojunction PV subcell with the underlying copper phthalocyanine (CuPc) and a bulk heterojunction PV subcell with blended poly(3-hexylthiophene-2,5-diyl) (P3HT). In comparison with the conventional tandem structure, the omission of the semitransparent intercellular connection layer reduces the complexity of the device and the light loss. The enhanced short circuit current density (JSC = 8.63 mA/cm2) and power conversion efficiency (PCE) (2.79%) of the tandem structure are nearly the sum of those of the stand-alone cells of CuPc/PCBM (JSC = 2.09 mA/cm2, PCE = 0.43%) and P3HT:PCBM (JSC = 6.87 mA/cm2, PCE = 2.50%).
    Applied Physics Letters 02/2008; 92(8):083310-083310-3. · 3.52 Impact Factor
  • Journal of The Electrochemical Society - J ELECTROCHEM SOC. 01/2008; 155(1).
  • Wei He, D. S. H. Chan, Byung-Jin Cho
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    ABSTRACT: LaAlOx with a permittivity of 17 is fabricated successfully by ALD method. Enhanced deposition rate, improved uniformity and self-limiting behavior were observed for LaAlOx compare to La2O3 deposition. The mechanism behind improvement is proposed. ALD LaAlOx is found to be thermally stable up to 850°C anneal. Compared with Al2O3 blocking oxide control samples, the SONOS devices with LaAlOx blocking oxide demonstrate similar retention performance with much faster operation speed and better resistance to high operation voltage and high stress. The results indicate that LaAlOx is an attractive candidate as a blocking layer in SONOS type flash memory application.
    01/2008;
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    ABSTRACT: Insertion of gold nanoparticles (GNPs) layer between the indium tin oxide (ITO) and the photoactive polymer by simple spin-coating method is found to improve the photovoltaic effects of solar cells significantly. Cross sectional SEM image provides direct evidence that GNPs layer acts as a pure buffer layer in our devices. The improvement of the device performance is due to good transmission, high work function, good conductivity, and ultra-smooth surface roughness of GNPs layer. GNPs layer can act as a promising buffer layer to facilitate hole collection from polymer donor towards ITO anode.
    Chemical Physics Letters 01/2008; 453:73-76. · 2.15 Impact Factor
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    ABSTRACT: This letter reports on the low-frequency flicker-noise characteristics in fresh and electrically stressed pMOSFETs with thin strained-Si (~4 nm)/Si<sub>0.6</sub>Ge<sub>0.4</sub> (~4 nm) dual-quantum-well (DQW) channel architectures. Normalized power spectral density (NPSD) of I<sub>d</sub> fluctuations (S<sub>ID</sub>/I<sub>d</sub> <sup>2</sup>) in fresh DQW devices exhibits significant improvement (by >10<sup>2</sup>times) due to buried channel operation at low V<sub>g</sub>. At high V<sub>g</sub>, the NPSD enhancement reduces as carriers populate in the parasitic surface channel. Upon electrical stress, noise behavior in DQW devices was found to evolve from being carrier number-fluctuation dominated to mobility- fluctuation dominated. This was accompanied by the observation of a "less-distinct" buried-channel operation, indicating a potential stability issue of the Si/SiGe structure.
    IEEE Electron Device Letters 08/2007; · 2.79 Impact Factor