M. Umeno

Over 18% solar energy conversion to generation of hydrogen fuel; theory and experiment for efficient solar water splitting

Abstract Contemporary models are shown to significantly underestimate the attainable efficiency of solar energy conversion to water splitting, and experimentally a cell containing illuminated AlGaAs/Si RuO2/Ptblack is demonstrated to evolve H2 and O2 at record solar-driven water electrolysis efficiency. Under illumination, bipolar configured Al 0.15 Ga 0.85 As (E g =1.6 eV ) and Si (E g =1.1 eV ) semiconductors generate open circuit and maximum power photopotentials of 1.57 and 1.30 V, well suited to the water electrolysis thermodynamic potential: H 2 O → H 2 +1/2 O 2 ; E H 2 O °=E O 2 −E H 2 ; E H 2 O °(25° C )=1.229 V . The EH2O°/photopotential matched semiconductors are combined with effective water electrolysis O2 or H2 electrocatalysts, RuO2 or Ptblack. The resultant solar photoelectrolysis cell drives sustained water splitting at 18.3% conversion efficiencies. Alternate dual bandgap systems are calculated to be capable of attaining over 30% solar photoelectrolysis conversion efficiency.

Structural and optical properties of diamond and nano-diamond films grown by microwave plasma chemical vapor deposition

Abstract We compare structural and optical properties of microcrystalline and nanocrystalline diamond (MCD and NCD, respectively) films grown on mirror polished Si(100) substrates by microwave plasma chemical vapor deposition. The films were characterized by SEM, Raman spectroscopy, XRD, and AFM. Optical properties were obtained from transmittance and reflectance measurements of the samples in the wavelength range of 200–2000 nm. Raman spectrum of the MCD film exhibits a strong and sharp peak near 1335 cm −1 , an unambiguous signature of cubic crystalline diamond with weak non-diamond carbon bands. Along with broad non-diamond carbon bands, Raman spectra of NCD films show features near 1140 cm −1 , the intensity of which is significantly higher in the film grown at 600°C compared to the NCD film grown at higher temperature. The Raman feature near 1140 cm −1 is related to the calculated phonon density of states of diamond and has been assigned to nanocrystalline or amorphous phase of diamond. XRD patterns of the MCD film show sharp peaks and NCD films show broad features, corresponding to cubic diamond. The rms surface roughness of the films was observed to be approximately 60 nm for MCD film that reduced substantially to 17 and 34 nm in the NCD films grown at 600 and 700°C, respectively. Taucs optical gap for the diamond film is found to be approximately 5.5 eV. NCD grown at 700°C has a high optical absorption coefficient in the whole spectral region and the NCD film grown at 600°C shows very high transmittance (∼78%) in the near IR region, which is close to that of diamond. This indicates that the NCD film grown at 600°C has the potential for applications as optical windows since its surface roughness is significantly low as compared to the MCD film.

Improved characteristics of InGaN multiple-quantum-well light-emitting diode by GaN/AlGaN distributed Bragg reflector grown on sapphire

An InGaN multiple-quantum-well light-emitting diode (LED) containing a GaN/AlGaN distributed Bragg reflector has been grown on a sapphire substrate by metalorganic chemical vapor deposition. Comparing with the conventional LED, the output power has been improved from 79 to 120 μW under 20 mA direct current biasing condition and the external quantum efficiency has been also improved from 0.16% to 0.23% under 10 mA dc current.

Effects of annealing on Ti, Pd, and Ni/n-Al/sub 0.11/Ga/sub 0.89/N Schottky diodes

Schottky diodes of Ti, Pd, and Ni/n-Al/sub 0.11/Ga/sub 0.89/N have been fabricated and the barrier heights were measured to be 0.60, 0.95 and 0.97 eV using current-voltage (I-V) measurements and to be 0.67, 1.15 and 1.22 eV using capacitance-voltage (C-V) measurements. Annealed Schottky diodes are show higher I-V and C-V barrier heights when compared with as-deposited Ti Schottky diodes except high temperature annealed (450/spl deg/C/30 min-500/spl deg/C/1 hr) Ti Schottky diodes. The I-V barrier height of Ti/n-Al/sub 0.11/Ga/sub 0.89/N increases up to the annealing temperature 350/spl deg/C/5 min and it decreased for higher annealing temperatures. The C-V barrier height increases up to the annealing temperature 150/spl deg/C/5 min for Ti (1.63 eV), 250/spl deg/C/5 min for both Pd (1.68 eV) and Ni (1.53 eV) Schottky diodes respectively. The increase of barrier heights for low temperature annealing is due to intimate contact between metal and semiconductor. Rectifying behavior has been observed up to the annealing temperature 450/spl deg/C/1 hr for Ni/n-Al/sub 0.11/Ga/sub 0.89/N and 500/spl deg/C/1 hr for both Ti and Pd/n-Al/sub 0.11/Ga/sub 0.89/N Schottky diodes. An increase of surface average roughness has been observed for the annealed Pd and Ni Schottky diodes except Ti Schottky diodes. Al/sup 0.11/Ga/sub 0.89/N surface behaves more like ceramic with both Pd and Ni than semiconductor.

Solar cells based on carbon thin films

Abstract Carbon as an alternative material for low-cost and high-efficiency solar cell is attempted. Thin films of semiconducting n- (p-doped) and p- (un-doped) type carbon have been deposited on various substrates by ion beam sputtering and pyrolysis, using camphor as a natural carbon precursor. The optical gap of carbon has been found to be around 1.05 and

Investigation of the effect of sol processing parameters on the photoelectrical properties of dye-sensitized TiO2 solar cells

Abstract Sol-gel spin-coating method has been explored to prepare nano-crystalline TiO2 films for dye-sensitized TiO2 solar cell application. Effects of evaporation temperature at which the sol is concentrated and addition of polyethelene glycol (PEG) to the sol at different concentrations on film thickness, surface morphology and solar cell performance are studied. Conversion efficiency is found to increase with PEG addition up to 40 % by weight of TiO2 and then decrease with further addition due to the development of cracks in the films. Higher conversion efficiency has been obtained with the sol concentrated at 40°C than at room temperature (30°C) after which it is found to decrease with increase in temperature due to higher degree of agglomeration caused by faster rate of evaporation. Further, it is observed that though the photocurrent and efficiency gradually increase with thickness at relatively lower thicknesses, a saturation in the photocurrent occurs at higher thicknesses.

InGaN multiple-quantum-well green light-emitting diodes on Si grown by metalorganic chemical vapor deposition

We report the characteristics of InGaN multiple-quantum-well (MQW) green light-emitting diodes (LEDs) on Si (111) substrates. The MQW LEDs were grown on Si by metalorganic chemical vapor deposition using Al0.27Ga0.73N/AlN intermediate layers. The LED on Si showed an operating voltage of 7 V, a series resistance of 100 Ω, an optical output power of 20 μW, and a peak emission wavelength of 505 nm with a full width at half maximum of 33 nm at 20 mA drive current. The optical output power was half as compared to that of green LED on sapphire. The LED also exhibited a stable operation over 500 h under automatic current control (20 mA) condition at 27 °C.

Spectroscopic properties of nitrogen doped hydrogenated amorphous carbon films grown by radio frequency plasma-enhanced chemical vapor deposition

Nitrogen doped hydrogenated amorphous carbon thin films have been deposited by rf plasma-enhanced chemical vapor deposition using CH4 as the source of carbon and with different nitrogen flow rates (N2/CH4 gas ratios between 0 and 3), at 300 K. The dependence modifications of the optical and the structural properties on nitrogen incorporation were investigated using different spectroscopic techniques, such as, Raman spectroscopy, Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, ultraviolet-visible (UV-VIS) spectroscopy, electron spin resonance (ESR), photoluminescence (PL) and spectroscopic ellipsometry (SE). Raman spectroscopy and IR absorption reveal an increase in sp2-bonded carbon or a change in sp2 domain size with increasing nitrogen flow rate. It is found that the configuration of nitrogen atoms incorporated into an amorphous carbon network gradually changes from nitrogen atoms surrounded by three (σ bonded) to two (π bonded) neighboring carbons with increasing nitrogen flo...

The infrared optical functions of AlxGa1−xN determined by reflectance spectroscopy

Infrared reflectivity measurements have been carried out on samples with structures of GaN/sapphire and AlxGa1−xN/GaN/sapphire as well as sapphire substrates. Analyses of the reflectance data of sapphire using the Kramers–Kronig technique and fitting of the reflectance spectra of GaN and AlxGa1−xN samples using analytical expressions have been made. The high-frequency dielectric constant e∞ and the transverse phonon frequency ωTO, are found to vary from 5.15 to 4.2 and from 559.7 to 586.4 cm−1, respectively, when the composition x is varied from 0 to 0.35 at room temperature. The E2 mode, which arises from the disordered state of the alloys, has been observed in the reflectivity spectrum of AlxGa1−xN, and the intensity of the peak is enhanced by increasing the Al content.

Low-threshold continuous-wave room-temperature operation of AlxGa1-xAs/GaAs single quantum well lasers grown by metalorganic chemical vapor deposition on Si substrates with SiO2 back coating

We demonstrate the first room‐temperature low‐threshold continuous‐wave (cw) operation of Al0.3Ga0.7As/GaAs single quantum well (SQW) heterostructure lasers grown by metalorganic chemical vapor deposition (MOCVD) on Si substrates using techniques of SiO2 back coating and thermal cycle annealing. The all‐MOCVD‐grown SQW lasers on GaAs/Si with etch pit density of 1.5× 107 cm−2 have threshold current as low as 55 mA (1.41 kA/cm2) under cw at room temperature. The SiO2 back coating is effective to obtain excellent current‐voltage characteristics. Thermal cycle annealing is also found to improve the crystallinity of GaAs/Si and to contribute to room‐temperature cw operation of the lasers on Si substrates.