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  • 1
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 1795-1797 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this letter, we present a simplified evaluation method for light-biased photoconductance decay measurements. The measured effective lifetime is shown to be a differential quantity τeff,d, which may differ significantly from the actual effective lifetime τeff. However, the actual effective lifetime can be approximated by integrating τeff,d directly over the incident power density of the bias light. The quality of the approximation depends mainly on the surface recombination velocity and the wavelength of the used bias light. However, the inaccuracy remains well below 10% for most practical cases. © 1997 American Institute of Physics.
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  • 2
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The photocarrier dynamics in pure nonhydrogenated amorphous silicon (a-Si) have been studied with subpicosecond resolution using pump-probe reflectivity measurements. The photocarrier lifetime increases with the annealing temperature from 1 ps for as-implanted a-Si to 11 ps for a-Si annealed at 500 °C. The lifetime in annealed a-Si can be returned to the as-implanted level by ion irradiation. These observations indicate that a-Si can accommodate a variable number of defect-related trapping and recombination centers. The saturated defect density in as-implanted a-Si is estimated to be ≈1.6 at. %. Comparison with Raman spectroscopy suggests that various kinds of structural defects are present in a-Si.
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  • 3
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Polycrystalline silicon wafers have been subjected to annealing (700 °C, 1 h) and to a hydrogen plasma (350 °C, 30 min) during the processing of solar cells. The annealing treatment enhances the bulk minority-carrier recombination lifetime by 19%, presumably by impurity gettering. The plasma treatment improves the lifetime by 26%; hydrogen passivation accounts for at least 2/3 of this improvement. Gettering and passivation are found to be complementary: application of both treatments results in a 43% increase in lifetime compared to standard.
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 1230-1232 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The diffusivity and solubility of Cu impurities have been measured in different structural states of amorphous Si (a-Si) formed by MeV Si implantation. The 2.2-μm-thick a-Si layers were first annealed (structurally relaxed) at 500 °C and then implanted with 200 keV Cu ions, returning a 300-nm-thick surface layer to the as-implanted state. After diffusion at temperatures in the range 150–270 °C, we observe solute partitioning at a sharp phase boundary between the annealed and Cu-implanted layers, the partition coefficient being as large as 8.2±1.3. The diffusion coefficient in annealed a-Si is 2–5 times larger than in as-implanted a-Si, with activation energies of 1.39±0.15 and 1.25±0.04 eV, respectively. The data show quite strikingly the role which defects can play in the a-Si structure.
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 1097-1099 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that amorphous silicon can be transformed to monocrystalline silicon via an explosive epitaxial crystallization process induced by pulsed laser irradiation. 370-nm-thick amorphous Si layers, buried beneath a 130-nm-thick crystalline surface layer, were irradiated with a 32 ns ruby laser pulse. Real-time reflectivity measurements indicate that internal melting can be initiated at the amorphous-crystalline interface, immediately followed by explosive crystallization of the buried amorphous Si layer. Channeling and cross-sectional transmission electron microscopy reveal that explosive crystallization proceeds epitaxially with formation of twins extending into the sample. The crystal growth velocity is determined to be 16.2±1.2 m/s, close to the fundamental limit for crystalline ordering at a liquid Si/Si(100) interface.
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  • 6
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The efficiency of silicon solar cells can be increased by reducing the recombination of minority carriers at the backsurface of a cell. This can be achieved by application of a low-high junction, commonly called a backsurface field (BSF). The dependence of the effective backsurface recombination velocity Seff of a BSF on the BSF doping profile and the base doping concentration was studied experimentally. The doping profiles were analyzed using sheet resistance measurements, stripping Hall measurements, and secondary ion mass spectrometry. The effective recombination velocity was obtained from photoconductivity decay probed with microwaves on symmetrical p+pp+ structures, and from photocurrent decay measurements on n+pp+ solar cells. The measured values of Seff were compared with calculated values, based on the measured doping profiles. Both from photoconductivity and photocurrent decay measurements, Seff was found to decrease with an increasing difference in acceptor concentration between the BSF and the base of the solar cell. The measured values of Seff, obtained with the two experimental techniques, are in agreement with each other. © 1995 American Institute of Physics.
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  • 7
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ion implantation, employing Si, Ar, and Cu ions in the energy range from 275 to 600 keV, was used to form amorphous silicon layers buried in a crystalline matrix. Different layer geometries were produced, with 150–620-nm-thick amorphous layers, separated from the surface by 120–350-nm-thick crystalline layers. Crystallization of the amorphous layers was induced by 32-ns pulsed ruby laser irradiation. Real-time reflectivity and conductivity measurements indicate that internal melting can be initiated at the amorphous-crystalline interface, immediately followed by explosive crystallization of the buried layer. Channeling and cross-section transmission electron microscopy reveal that in both Si(100) and Si(111) samples explosive crystallization proceeds epitaxially with twin formation, the twin density being higher in Si(111) than in Si(100). The measured crystal growth velocities range from 15 to 16 m/s, close to the fundamental limit for crystalline ordering at a Si liquid-crystalline interface. Computer modeling of heat flow and phase transformations supports the experimental data.
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 437-439 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The density of amorphous Si has been measured. Multiple Si implants, at energies up to 8.0 MeV, were made through a contact mask to produce alternating amorphous/crystalline Si stripes with amorphous thicknesses up to ∼5.0 μm. For layers up to 3.4 μm (5 MeV), the amorphous Si is constrained laterally and deforms plastically. Above 5 MeV, plastic deformation of the surrounding crystal matrix is observed. Height differences between the amorphous and crystalline regions were measured for as-implanted, thermally relaxed, and partially recrystallized samples using a surface profilometer. Combined with ion channeling measurements of the layer thickness, amorphous Si was determined to be 1.8±0.1% less dense than crystalline Si (4.90×1022 atom/cm3 at 300 K). Both relaxed and unrelaxed amorphous Si show identical densities within experimental error (〈0.1% density difference).
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 2792-2794 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present a new technique for boron (B) doping of silicon. In this letter we show that a B doping profile of more than 2 μm thickness with a maximum active doping concentration of 3×1019 cm−3 can be formed by a fast (〈1 min) alloying process at a temperature of 850 °C. Thick-film aluminum is used to obtain an alloying and epitaxial regrowth process in accordance with the Al–Si phase diagram. The atomic concentration profile of the B- and Al-doped Si layer was determined by secondary ion mass spectrometry. The active doping concentration was measured with an automatic electrochemical capacitance/voltage profiler. By the addition of B to the Al paste, the epitaxially regrown layer is doped with B and Al to the solid solubility of these elements in Si at the particular regrowth temperature. The applicability of this fast low temperature B doping process in Si solar cell processing is discussed. © 1994 American Institute of Physics.
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  • 10
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In modern production schemes for Si solar cells, defect passivation and impurity gettering are often used to improve material quality and thereby cell efficiency. These processes generally alter the spatial uniformity of minority-carrier transport parameters over the wafer and may result in minority-carrier lifetime and diffusion length variations both lateral and in depth. In polycrystalline Si these spatial dependences are already present due to the nature of the material. We present an extension of the photocurrent decay method to determine the diffusion length in a three-dimensionally resolved fashion. From a single photocurrent decay curve the back-surface recombination velocity, the average minority-carrier diffusion length, and an asymmetry factor which qualitatively describes the depth dependence of the diffusion length are determined. This is done using the three observables: quantum efficiency, fundamental decay time, and intercept of the extrapolated decay curve with the time-zero axis. Lateral resolution is obtained by focusing the light beam to a small spot on the cell and measuring the current decay curve as a function of position on the cell. It is shown that light-pulse durations longer than the minority-carrier lifetime and wavelengths longer than 950 nm are required. These conditions are met by using modulated wavelength-tunable light from a Ti:sapphire laser. Measurements on monocrystalline cells show that the decay time is independent of wavelength and light-pulse duration, as predicted by theory. Furthermore, the intercept with the time-zero axis was shown to increase with increasing pulse duration and wavelength. Measurements on a set of polycrystalline Si cells were performed showing that gettering treatments during cell production result in depth-dependent lifetimes.
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