M. L. Lee

National Cheng Kung University, Tainan, Taiwan, Taiwan

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Publications (51)112.52 Total impact

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    ABSTRACT: Hydrogen generation through water splitting by n-InGaN working electrodes with bias generated from GaAs solar cell was studied. Instead of using an external bias provided by power supply, a GaAs-based solar cell was used as the driving force to increase the rate of hydrogen production. The water-splitting system was tuned using different approaches to set the operating points to the maximum power point of the GaAs solar cell. The approaches included changing the electrolytes, varying the light intensity, and introducing the immersed ITO ohmic contacts on the working electrodes. As a result, the hybrid system comprising both InGaN-based working electrodes and GaAs solar cells operating under concentrated illumination could possibly facilitate efficient water splitting.
    Optics Express 11/2013; 21 Suppl 6:A991-6. · 3.55 Impact Factor
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    ABSTRACT: Hydrogen generation through direct photoelectrolysis of water was studied using photoelectrochemical (PEC) cells made of Mn-doped GaN photoelectrodes. In addition to its absorption of the ultraviolet spectrum, Mn-doped GaN photoelectrodes could absorb photons in the visible spectrum. The photocurrents measured from PEC cells made of Mn-doped GaN were at least one order higher than those measured from PEC cells made of undoped GaN-working electrodes. Under the visible light illumination and a bias voltage below 1.2 V, the Mn-doped GaN photoelectrodes could drive the water splitting reaction for hydrogen generation. However, hydrogen generation could not be achieved under the same condition wherein undoped GaN photoelectrodes were used. According to the results of the spectral responses and transmission spectra obtained from the experimental photoelectrodes, the enhanced photocurrent in the Mn-doped GaN photoelectrodes, compared with the undoped GaN photoelectrodes, was attributable to the Mn-related intermediate band within the band gap of GaN that resulted in further photon absorption.
    Optics Express 09/2012; 20 Suppl 5:A678-83. · 3.55 Impact Factor
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    ABSTRACT: In this study, we demonstrated photoelectrochemical (PEC) hydrogen generation using p-GaN photoelectrodes associated with immersed finger-type indium tin oxide (IF-ITO) ohmic contacts. The IF-ITO/p-GaN photoelectrode scheme exhibits higher photocurrent and gas generation rate compared with p-GaN photoelectrodes without IF-ITO ohmic contacts. In addition, the critical external bias for detectable hydrogen generation can be effectively reduced by the use of IF-ITO ohmic contacts. This finding can be attributed to the greatly uniform distribution of the IF-ITO/p-GaN photoelectrode applied fields over the whole working area. As a result, the collection efficiency of photo-generated holes by electrode contacts is higher than that of p-GaN photoelectrodes without IF-ITO contacts. Microscopy revealed a tiny change on the p-GaN surfaces before and after hydrogen generation. In contrast, photoelectrodes composed of n-GaN have a short lifetime due to n-GaN corrosion during hydrogen generation. Findings of this study indicate that the ITO finger contacts on p-GaN layer is a potential candidate as photoelectrodes for PEC hydrogen generation.
    Optics Express 03/2012; 20 Suppl 2:A190-6. · 3.55 Impact Factor
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    ABSTRACT: The mechanism responsible for the efficiency droop in AlGaInP based vertically-structured red light-emitting diodes (LEDs) is investigated using dynamic measurement techniques. Short electrical pulses (~100ps) are pumped into this device and the output optical pulses probed using high-speed photo-receiver circuits. From this, the internal carrier dynamic inside the device can be investigated by use of the measured electrical-to-optical (E-O) impulse responses. Results show that the E-O responses measured under different bias currents are all invariant from room temperature to ~100°. This is contrary to most results reported for AlGaInP based red LEDs, which usually exhibit a shortening in the response time and degradation in output power with the increase of ambient temperature. According to these measurement results and the extracted fall-time constants of the E-O impulse responses, the origin of the efficiency droop in our vertical LED structure, which has good heat-sinking, is not due to thermally induced carrier leakage, but rather should be attributed to defect recombination and the saturation of spontaneous recombination processes.
    Proc SPIE 02/2012;
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    ABSTRACT: Distinct temperature-dependent dynamic behaviors of GaN-based blue light-emitting diodes (LEDs) are observed by use of the very-fast electrical-optical pump-probe technique. Our static and impulse response measurement results indicate that the behaviors of internal carrier dynamics under different ambient temperatures can be classified into three regimes covering a wide range of bias current densities (20-2000 A/cm2). The first regime is when the bias current density ranges from low to moderate (20-100 A/cm2). The measured external quantum efficiency (EQE) degrades dramatically from 57 to 44%, and the measured waveform and extracted time constants of measured impulse responses are invariable from room temperature (RT) to 200 °C, which indicates that the carrier leakage is not an issue for the observed droop phenomenon. When the bias current density further increases to near 1 kA/cm2, the droop phenomenon are mitigated (44 to 24%). However, a significant shortening of the measured impulse response happens under 200 °C operation due to the device-heating effect. This phenomenon is diminished when the bias current densities are further increased to over 1 kA/cm2, due to the screening of the piezoelectric field. The extracted time constants can also be used to explain the droop phenomenon in GaN LED under high bias currents.
    IEEE Photonics Journal 01/2012; 4(5):1870-1880. · 2.36 Impact Factor
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    ABSTRACT: An n-GaN photoelectrochemical (PEC) cell with immersed finger-type indium tin oxide (ITO) ohmic contacts was demonstrated in the present study to enhance the hydrogen generation rate. The finger-type ITO ohmic contacts were covered with SiO₂ layers to prevent the PEC cell from generating leakage current. Using a 1M NaCl electrolyte and external biases, the typical photocurrent density and gas generation rate of the n-GaN working electrodes with ITO finger contacts were found to be higher than those with Cr/Au finger contacts. The enhancement in photocurrent density or gas generation rate can be attributed to the transparent ITO contacts which allowed the introduction of relatively more photons into the GaN layer. No significant corrosion was observed in the ITO layer after the PEC process compared with the Cr/Au finger contacts which were significantly peeled from the GaN layer. These results indicate that the use of n-GaN working electrodes with finger-type ITO ohmic contacts is a promising approach for PEC cells.
    Optics Express 11/2011; 19 Suppl 6:A1196-201. · 3.55 Impact Factor
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    ABSTRACT: The InGaN/sapphire-based photovoltaic (PV) cells with Al<sub>0.14</sub>Ga<sub>0.86</sub>N/In<sub>0.21</sub>Ga<sub>0.79</sub>N superlattice structures that serve as absorption layers were grown on patterned sapphire substrates (PSSs). Under global air-mass 1.5 conditions, the short-circuit current density, the open-circuit voltage, and the fill factor obtained from the PV cells were 1.21 mA/cm<sup>2</sup>, 2.18 V, and 0.65, respectively, corresponding to a conversion efficiency of 1.71%. Compared with PV devices grown on flat sapphire substrates, the photocurrent of PSS-grown PV devices was enhanced by 26%. The improved PV performance was attributable to the positive effects of the PSS on the material quality.
    IEEE Electron Device Letters 05/2011; · 2.79 Impact Factor
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    ABSTRACT: The effect of growth pressure of underlying undoped GaN(u-GaN) layer on the electrical properties of GaN-based light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSS) is evaluated. The electrostatic discharge (ESD) endurance voltages could increase from 4000 to 7000 V when the growth pressure of u-GaN layers is increased from 100 to 500 torr, while the forward voltages and light output powers remain almost the same. Poor ESD endurance ability could be attributed to the underlying GaN layer grown under relative low pressure, which leads to significant surface pits. This could be further attributed to the imperfect coalescence of crystal planes above the convex sapphire patterns. The pits are associated with TDs behaving as a leakage path to degrade electrical performance.
    IEEE Photonics Technology Letters 01/2011; 23:968-970. · 2.04 Impact Factor
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    ABSTRACT: Electrical-optical pump-probe is used to investigate GaN blue LEDs under different temperatures. Measurement result indicates that under moderate current density (∼200A/cm2) piezoelectric field induced carrier-escaping cannot be neglected and is responsible for the observed efficiency-droop.
    01/2011;
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    ABSTRACT: GaN-based LEDs grown on Si-implanted GaN templates form air gaps beneath the active layer to enhance light- extraction efficiency. GaN-based epitaxial layers grown on selec- tive Si-implanted regions had lower growth rates compared with those grown on implantation-free regions, resulting in selective growth and the subsequent over the Si-implanted regions. Accord- ingly, air gaps were formed over the Si-implanted regions after the meeting of laterally growing GaN facet fronts. The experimental results indicate that the light-output power of the LEDs grown on the Si-implanted GaN templates was enhanced by 36% compared with conventional LEDs. This enhancement in output power was attributed mainly to the air gaps, which led to a higher escape probability for the photons. Index Terms—Air gaps, lateral growth, Si-implanted.
    IEEE Electron Device Letters 01/2011; 32(10):1400-1402. · 2.79 Impact Factor
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    ABSTRACT: To reduce light reflection and enlarge the effective reaction surface area at the n-GaN/electrolyte interface, n-GaN epitaxial layers with naturally textured surface are utilized. The layers are combined with ohmic contacts on n-GaN to form working electrodes that generate hydrogen by direct photoelectrolysis of water. Although the surface reflection on the naturally textured n-GaN samples is lower than that on n-GaN epitaxial layers with flat surface, our results reveal that the photocurrent and gas generation rates obtained from the naturally rough n-GaN samples are lower than those from the flat samples. The results can be attributed to the fact that the rough n-GaN surface caused by dense surface pits leads to significant recombination of photogenerated carriers with charged defects; this occurs before carriers reach the ohmic contacts, thereby resulting in lower and . Related analyses have been performed and presented in this paper to initially explain the possible mechanism.
    Journal of The Electrochemical Society. 11/2010; 157(12):H1106-H1109.
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    ABSTRACT: Edge-emitting c-plane GaN/sapphire-based light-emitting diodes (LEDs) sandwiched by two dielectric/metal hybrid reflectors on both sapphire and GaN surfaces were studied to determine their light emission polarization. The hybrid reflectors comprised dielectric multiple thin films and a metal layer. The metal layers of Au or Ag used in this study were designed to enhance the polarization ratio from S-polarization (transverse electric wave, TE) to P-polarization (transverse magnetic wave, TM). The two sets of optimized dielectric multi thin films served as matching layers for wide-angle incident light on both sapphire and GaN surfaces. To determine which reflector scheme would achieve a higher polarization ratio, simulations of the reflectance at the hybrid reflectors on sapphire (or GaN) interface were performed before the fabrication of experimental LEDs. Compared with conventional c-plane InGaN/GaN/sapphire LEDs without dielectric/metal hybrid reflectors, the experimental LEDs exhibited higher polarization ratio (ITE-max/ITM-max) with r = 2.174 ( ∼ 3.37 dB) at a wavelength of 460 nm. In contrast, the original polarized light (without dielectric/metal hybrid reflectors) was partially contributed (r = 1.398) by C-HH or C-LH (C band to the heavy-hole sub-band or C band to the crystal-field split-off sub-band) transitions along the a-plane or m-plane direction.
    Journal of Applied Physics 11/2010; 108(11):113102-113102-5. · 2.21 Impact Factor
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    ABSTRACT: We demonstrate a novel type of linear cascade green light-emitting diode (LED) arrays as a light source for in-car or harsh environment plastic optical fiber (POF) communications. To further enhance its dynamic and static performance, an InGaN layer is inserted between an n-type GaN cladding layer and InGaN-GaN multiple quantum wells as an efficient current spreading layer. Compared with the control device without that layer, our three-LED cascade array demonstrates a smaller turn-on voltage (9.3 versus 11 V at 20 mA) and a larger output power (25.5 versus 22.5 mW at 180 mA), corresponding to an enhancement of around 31% in wall-plug efficiency. Furthermore, under the constant voltage bias of an in-car battery (12 V), our three-LED array exhibits an electrical-to-optical 3-dB bandwidth (100 versus 40 MHz) performance superior to that of the control device. Even under high-temperature dynamic operation, we observe that the InGaN insertion layer gives strong enhancement of modulation speed with negligible degradation of the output power, unlike the red resonant-cavity LEDs conventionally used for POF. We achieve 200-Mb/s error-free transmission at 200°C which is the highest operation temperature among all the reported high-speed LEDs.
    IEEE Photonics Technology Letters 08/2010; · 2.04 Impact Factor
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    ABSTRACT: InGaN/sapphire-based p-i-n type photovoltaic (PV) devices were shown to have Al0.14Ga0.86N/In0.21Ga0.79N heterostructures that enhance the extraction of photogenerated carriers from active layers. With an appropriately increased barrier height in AlGaN/InGaN absorption layers, PV devices exhibit lower RS despite the increase in conduction-band discontinuity compared with GaN/InGaN superlattice absorption layers. This improvement can be attributed to polarization-induced electric fields enhanced by the incorporated aluminum in barrier layers. The enhancement is beneficial to increase built-in electric fields. Subsequently, the photogenerated carriers can escape more easily from recombination or scattering centers. Under 1 sun air-mass 1.5 standard testing conditions, the Al0.14Ga0.86N/In0.21Ga0.79N PV device exhibits high VOC (2.10 V) as well as an enhanced fill factor (0.66) and JSC (0.84 mA/cm2) corresponding to a power conversion efficiency of 1.16%.
    Applied Physics Letters 07/2010; 97(2):021113-021113-3. · 3.52 Impact Factor
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    ABSTRACT: We demonstrate cascade green light-emitting-diodes, which greatly release trade-off between output-power and speed and exhibits strong modulation-speed enhancement with negligible output-power degradation from room-temperature to 200°C operation. 200 Mbit/sec error-free transmission at 200°C can be achieved.
    Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC); 04/2010
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    ABSTRACT: We demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts and textured Ga-doped ZnO (GZO) contact layer to serve as the n- and p-type electrode pads, respectively. Compared with the conventional LEDs with flat surface and Cr/Au metal contacts, the nonalloyed Ag/Cr/Au contacts used in the present experimental LEDs play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads. Enhancement of light output power observed from the experimental LEDs is also due to the textured GZO layer that can disperse the angular distribution of photons at the GZO/air interface. With an injection current of 20 mA, the output power of experimental LEDs can be improved markedly by a magnitude of 30% compared with conventional GaN-based LEDs.
    Applied Physics Letters 03/2010; 96(13):133504-133504-3. · 3.52 Impact Factor
  • Journal of The Electrochemical Society - J ELECTROCHEM SOC. 01/2010; 157(2).
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    ABSTRACT: We demonstrated a GaN-based p-i-n photodiode by inserting a thin low-temperature-grown-GaN layer between p-type Al0.2Ga0.8N window and intrinsic GaN layer. As compared with control device, our demonstrated one can achieve 3 fold bandwidth-efficiency-products improvement.
    05/2009;
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    ABSTRACT: In this study, indium tin oxide (ITO) or ITO/ZnO films, which were prepared by magnetron sputtering, were deposited onto p-GaN epitaxial films to form ultraviolet photodetectors (PDs). The ITO/ZnO/p-GaN heterojunction PDs behave like the p-i-n photodiodes, which characteristically exhibit low dark current, as opposed to the ITO/p-GaN PDs, which exhibit a marked bias-dependent dark current. The zero-bias rejection ratio can be improved up to 4×105 due to a further reduction in the dark current compared to the ITO/p-GaN PDs. When the incident wavelength is 360 nm, the ITO/ZnO/p-GaN heterojunction PD exhibits a zero-bias photocurrent/dark current ratio and a responsivity of approximately 8×104 and 0.015 A/W, respectively.
    Applied Physics Letters 01/2009; 94(1):013512-013512-3. · 3.52 Impact Factor
  • Journal of The Electrochemical Society - J ELECTROCHEM SOC. 01/2009; 156(8).

Publication Stats

373 Citations
112.52 Total Impact Points

Institutions

  • 2003–2012
    • National Cheng Kung University
      • • Advanced Optoelectronic Technology Center
      • • Institute of Electro-Optical Science and Engineering
      • • Department of Electrical Engineering
      • • Institute of Microelectronics
      Tainan, Taiwan, Taiwan
  • 2006–2010
    • Southern Taiwan University of Science and Technology
      • Department of Electrical Engineering
      臺南市, Taiwan, Taiwan
  • 2002–2010
    • National Central University
      • • Department of Electrical Engineering
      • • Department of Physics
      Taoyuan City, Taiwan, Taiwan