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Adachi, Sadao
概要:
application/pdf<br />Journal Article<br />The room-temperature thermal conductivity of semiconductor alloys is analyzed using a simplified\nmodel of the alloy-disorder scattering. Good agreement is achieved between the present
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model and\npublished experimental data on various group-IV and III-V semiconductor alloys. A complete set\nof alloy-disorder parameters are estimated, which makes it possible to calculate the lattice thermal\nconductivity for optional composition of III-V semiconductor alloys, including III-N alloys. An\nordering effect is also examined for the explanation of some intermetallic and semiconductor\ncompounds like CuAu and SiC.
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| 2.
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Adachi, Sadao ; Oi, Mitsuru
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application/pdf<br />Journal Article<br />The optical properties of porous silicon (PSi) photoetched in aqueous HF/I2 so
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lution are\ninvestigated using spectroellipsomety (SE), electroreflectance (ER), photovoltage (PV),\nphotoconductivity (PC), photoluminescence (PL), and Fourier transform infrared (FTIR)\nspectroscopy. The PSi layers were formed in a HF/I2 solution on n-Si substrates under Xe lamp\nillumination. The SE ε(E) and related data show an interference oscillation in the region below\nE～3 eV, where the PSi material is nearly transparent. The PV and PC spectra reveal three\nindividual peaks A, B, and C at ～1.2, ～1.7, and ～2.5 eV, respectively, arising from the PSi layer\nitself. Peak C is also observed in the ER spectrum, together with a broadened E1 peak at ～3.4 eV.\nChange in the fundamental-absorption-edge nature (EgX) from the indirect gap in crystalline silicon\nto the quasidirect gap in PSi is found in the PV and PC spectra. The PL spectrum shows a broad\npeak at ～2.0 eV(B). Peaks A, B, and C observed in the PSi layer may originate from the nondirect\noptical transitions at and above the lowest absorption edges EgX (A and B) and EgL (C). The\nquantum-mechanical size effect, i.e., a relaxation of the momentum conservation, makes possible\nthe nondirect or quasidirect transitions at and above EgX and EgL in porous materials. The FTIR data\nsupport that the PL emission is due to the surface-sensitive quantum confinement effect.
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Morota, Hiroaki ; Adachi, Sadao
概要:
application/pdf<br />Journal Article<br />Chemically cleaned GaP(001) surfaces in aqueous HF solutions have been studied
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using\nspectroscopic ellipsometry (SE), ex situ atomic force microscopy (AFM), x-ray photoelectron\nspectroscopy (XPS), wettability, and photoluminescence (PL) measurements. The SE data clearly\nindicate that the solutions cause removal of the native oxide film immediately upon immersing the\nsample (?1 min). The SE data, however, suggest that the native oxide film cannot be completely\netch-removed. This is due to the fact that as soon as the etched sample is exposed to air, the oxide\nstarts to regrow. The SE estimated roughness is ～1 nm, while the AFM roughness value is\n～0.3 nm. The XPS spectra confirm the removal of the native oxide and also the presence of regrown\noxide on the HF-etched GaP surface. The wettability measurements indicate that the HF-cleaned\nsurface is hydrophobic, which is in direct contrast to those obtained from alkaline-cleaned surfaces\n(hydrophilic). A slight increase in the PL intensity is also observed after etching in aqueous HF solutions.
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Xu, Yan Kai ; Adachi, Sadao
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application/pdf<br />Journal Article<br />The formation of yellow-light-emitting porous silicon (PSi) layers in a HF solution with adding an oxidizing agent FeCl3 is presented. The PSi layers are formed by photoetching under Xe lamp
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illumination. The photoluminescence (PL) intensity is strongly dependent on the FeCl3 concentration and shows a maximum at x~25 wt % [50 wt % HF:(x wt % FeCl3 in H2O)=1:1]. The surface topography as characterized by atomic force microscopy reveals features on the order of 20-100 nm with a root-mean-squares roughness of <=2 nm. The Fourier-transform infrared spectroscopy shows a new absorption peak at ~1100 cm-1, which is assigned to the surface oxide stretching mode and grows larger with increasing etching time. The stain etched samples also show PL emission, but they are synthesized only at higher x concentrations (>=20 wt %). The PSi formation mechanism can be explained with the aid of a surface energy-band diagram of n-type silicon in the HF/FeCl3 electrolyte.
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Ozaki, Shunji ; Adachi, Sadao
概要:
application/pdf<br />Journal Article<br />Optical-absorption and photoluminescence (PL) spectra have been measured on th
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e ternary\nchalcopyrite semiconductor AgInSe2 at T=13–300 K. The direct-band-gap energy Eg of AgInSe2\ndetermined from the optical absorption measurements shows unusual temperature dependence at\nlow temperatures. The resultant temperature coefficient ?Eg/?Tis found to be positive at T\n<125 K and negative above 125 K. The PL spectra show asymmetric emission bands peaking at\n～1.18 and ～1.20 eV at T=13 K, which are attributed to donor-acceptor pair recombinations\nbetween exponentially tailed donor states and acceptor levels. An energy-band scheme has been\nproposed for the explanation of the peculiar PL emission spectra observed in AgInSe2.
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| 6.
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Morota, Hiroaki ; Adachi, Sadao
概要:
application/pdf<br />Journal Article<br />Chemically cleaned GaP(001) surfaces in 25% NH4OH solution have been studied u
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sing\nspectroscopic ellipsometry (SE), ex situ atomic force microscopy (AFM), x-ray photoelectron\nspectroscopy (XPS), and wettability measurement techniques. The SE data clearly indicate that the\nsolution causes removal of the native oxide film immediately upon immersing the sample. The SE\ndata also indicate that when the native oxide film is completely etch removed, the resulting surface\nis still roughened. The estimated roughness thickness is ～1.2 nm, in excellent agreement with the\nAFM rms value (～1.2 nm). The XPS spectra confirm the removal of the native oxide from the GaP\nsurface. The XPS data also suggest a thin oxide overlayer, ～0.3 nm thick, on the etch-cleaned GaP\nsurface. The wettability measurements indicate that the as-degreased surface is hydrophobic, while\nthe NH4OH-cleaned surface is hydrophilic. This result is in direct contrast to those obtained from\nacid cleaned surfaces, which are usually hydrophobic. The origin of hydrophilicity may be singular\nand associated hydroxyl groups bonded on the GaP surface.
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Uchida, Kaoru ; Tomioka, Katsuhiro ; Adachi, Sadao
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application/pdf<br />Journal Article<br />Stable ultraviolet (UV) photoluminescence (PL) has been observed at room tempe
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rature in porous\nsilicon (PSi) fabricated by photoetching in aqueous alkali fluoride solutions. The aqueous solutions\nused are 1 M NaF and 1 M KF.They give an alkaline reaction caused by partial hydrolysis. The PL\npeaks at ～3.3 eV have a full width at half maximum of ～0.1 eV, which is much smaller than those\nreported previously (?0.5 eV). Spectral analyses suggest that both quantum confinement and\nsurface passivation effects enable the observation of UV emission in NaF- and KF-prepared PSi\nsamples.
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| 8.
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Tomioka, Katsuhiro ; Adachi, Sadao
概要:
application/pdf<br />Journal Article<br />A new method of fabricating porous silicon emitting in the ultraviolet (UV) sp
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ectral region is\npresented. This method uses photoetching in an aqueous salt (KF) solution. Strong UV\nphotoluminescence is observed at ～3.3 eV with a full width at a half maximum of ～0.1 eV, which\nis much narrower than those reported previously. Fourier transform infrared spectroscopy suggests\nthat the surface oxide produced during photoetching plays an important role in the UV emission of\nthe KF-prepared PSi.
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| 9.
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Tomioka, Katsuhiro ; Adachi, Sadao
概要:
application/pdf<br />Journal Article<br />The structural and optical properties of porous GaP have been studied by scann
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ing electron\nmicroscopy, spectroscopic ellipsometry, and photoluminescence (PL) spectroscopy. Porous GaP\nlayers were fabricated by anodic etching in HF:H2O:C2H5OH=1:1:2 electrolyte on n-type (100)\nand (111)A substrates. The morphology of the porous GaP layer is found to depend strongly on the\nsurface orientation. Apart from the red emission band at ～1.7 eV, a supra-band-gap (EgX) emission\nhas been clearly observed on the porous GaP (111)A sample. The anodic porous layer on the (100)\nsubstrate, on the other hand, has shown only the red emission at 300 K and both red and green\ndonor-acceptor pair emissions at low temperatures. The correlation between the PL properties and\nthe porous morphology is discussed. An optical transition model is also proposed for the explanation\nof the PL emission properties of the porous GaP samples.
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| 10.
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Miyazaki, Takayuki ; Takada, Kouhei ; Adachi, Sadao ; Ohtsuka, Kohji
概要:
application/pdf<br />Journal Article<br />GaN films have been deposited by reactive sputtering in nitrogen gas at pressu
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res from 0.08 to 2.70\nPa with and without the addition of hydrogen gas. X-ray diffraction (XRD), Fourier transform\ninfrared (FTIR), optical absorption, and photoluminescence (PL) spectroscopy have been used to\ncharacterize the sputter-deposited GaN films. The XRD pattern reveals that the GaN films deposited\nin nitrogen gas at pressures lower than 0.53 Pa are polycrystals with the (0001) texture (α-GaN),\nwhile those deposited at or above 1.07 Pa display mixed crystalline orientations or an\namorphous-like nature. The GaN:H films deposited in nitrogen/hydrogen mixed gas, on the other\nhand, show an amorphous or amorphous-like nature. The FTIR spectra indicate that the GaN:H\nfilms show peaks arising from hydrogen-related bonds at ～1000 and ～3200 cm−1, in addition to the\nGaN absorption band at ,555 cm−1. The optical absorption spectra at 300 K indicate the\nfundamental absorption edges at ～3.38 and ～3.7 eV for the highly oriented α-GaN and amorphous\nGaN:H films, respectively. PL emission has been observed from sputter-deposited a-GaN films at\ntemperatures below 100 K. The GaN:H films also show strong band-edge and donor-acceptor pair\nemissions. The PL emission in the GaN:H film may arise from crystalline GaN particles embedded\nin the amorphous GaN matrix.
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