Towards high efficiency nanowire solar cells

Defect engineering in photocatalytic materials has recently attracted substantial attention owing to the profound influence of defects on light absorption, charge separation, and interfacial reactions. However, the ambiguity regarding the role of defects in modifying the photocatalytic properties remains a long-standing problem. In this review, defect engineering in photocatalytic materials is comprehensively discussed with a focus on the classification, formation, and characterization of defects and their roles in photocatalytic systems. A systematic study of charge density, rate of charge recombination, and dissociative adsorption in defect-rich photocatalysts is also presented. Defects have the ability to alter the mechanisms of photocatalytic reactions, although the triggering of undesired thermodynamically favored back reactions by excess defects can lead to diminished photocatalytic activity. Furthermore, the critical roles of defects in photocatalytic degradation, H2 production, and bifunctional systems are reviewed. Finally, the challenges and future potential of defect engineering are considered.

Defect engineering in photocatalysis: formation, chemistry, optoelectronics, and interface studies

The III-V compound semiconductor, which has the advantage of wide bandgap and high electron mobility, has attracted increasing interest in the optoelectronics and microelectronics field. The poor electronic properties of III-V semiconductor surfaces resulting from a high density of surface/interface states limit III-V device technology development. Various techniques have been applied to improve the surface and interface quality, which cover sulfur-passivation, plasmas-passivation, ultrathin film deposition, and so on. In this paper, recent research of the surface passivation on III-V semiconductors was reviewed and compared. It was shown that several passivation methods can lead to a perfectly clean surface, but only a few methods can be considered for actual device integration due to their effectiveness and simplicity.

Brief Review of Surface Passivation on III-V Semiconductor

Ultraweak light detectors have wide-ranging important applications such as astronomical observation, remote sensing, laser ranging, and night vision. Current commercial ultraweak light detectors are commonly based on a photomultiplier tube or an avalanche photodiode, and they are incompatible with microelectronic devices for digital imaging applications, because of their high operating voltage and bulky size. Herein, we develop a memory phototransistor for ultraweak light detection, by exploiting the charge-storage accumulative effect in CdS nanoribbon. The memory phototransistors break the power law of traditional photodetectors and follow a time-dependent exponential-association photoelectric conversion law. Significantly, the memory phototransistors exhibit ultrahigh responsivity of 3.8 × 109 A W−1 and detectivity of 7.7 × 1022 Jones. As a result, the memory phototransistors are able to detect ultraweak light of 6 nW cm−2 with an extremely high sensitivity of 4 × 107. The proposed memory phototransistors offer a design concept for ultraweak light sensing devices. CdS nanostructures can enable memory based photodetection by charge-storage accumulative effect. Here, the authors report CdS nanoribbons-based memory phototransistors with high responsivity of 3.8 × 109 A/W and detectivity of 7.7 × 1022 Jones that can detect weak light of 6 nW/cm2.

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Memory phototransistors based on exponential-association photoelectric conversion law.

Nanowire photodetectors, which have the advantages of fast response and high photoelectric conversion efficiency, can be widely applied in various industries. However, the rich surface states result in large dark current and can hinder the development of high-performance nanowire photodetectors. In this paper, the influence and mechanism of sulfur surface passivation on the dark current of a single GaAs nanowire photodetector have been studied. The dark current is significantly reduced by about 30 times after surface passivation. We confirm that the origin of the reduction of dark current is the decrease in the surface state density. As a result, a single GaAs nanowire photodetector with low dark current of 7.18 × 10−2 pA and high detectivity of 9.04 × 1012 cmHz0.5W−1 has been achieved. A simple and convenient way to realize high-performance GaAs-based photodetectors has been proposed.

https://iopscience.iop.org/article/10.1088/1361-6528/aaa4d6/pdf

Analysis of the influence and mechanism of sulfur passivation on the dark current of a single GaAs nanowire photodetector.

Surface nitridation by hydrazine–sulfide solution, which is known to produce surface passivation of GaAs crystals, was applied to GaAs nanowires (NWs). We studied the effect of nitridation on conductivity and microphotoluminescence (μ-PL) of individual GaAs NWs using conductive atomic force microscopy (CAFM) and confocal luminescent microscopy (CLM), respectively. Nitridation is found to produce an essential increase in the NW conductivity and the μ-PL intensity as well evidence of surface passivation. Estimations show that the nitride passivation reduces the surface state density by a factor of 6, which is of the same order as that found for GaAs/AlGaAs nanowires. The effects of the nitride passivation are also stable under atmospheric ambient conditions for six months.

Nitride Surface Passivation of GaAs Nanowires: Impact on Surface State Density

Wet sulfur passivation of GaSb(1 0 0) surface for optoelectronic applications

InP(1 0 0) surface passivation with aqueous sodium sulfide solution

Modification of the surface chemical composition and electronic structure of the native-oxide-covered p-GaP(001) surface by treatment with sulfide solution is studied by high-resolution synchrotron photoemission spectroscopy (SXPS) and reflectance anisotropy spectroscopy (RAS). Treatment of the p-GaP(001) surface with a solution of ammonium sulfide in 2-propanol causes removal of the native oxide layer and formation of a passivating layer consisting mainly of gallium sulfides and sulfates. The latter species are formed due to oxidation of sulfides with hydroxyl groups existing in the solution. After such a treatment the downward surface band bending of about 0.5 eV remains essentially unaffected, while the electronic work function increases by 0.6 eV indicating modification of the surface dipole. As a result, the valence band maximum at the surface shifts to 5.6 eV. In reflectance anisotropy spectra the surface dipole modification produces an essential increase of the anisotropy signal in the spectral region of the direct optical transition point E1 indicating considerable increase of the near-surface electric field. This dipole-induced near-surface electric field remains essentially unchanged on air exposure of the sulfur-treated p-GaP(001) surface for at least 6 months.

Modification of the p-GaP(001) work function by surface dipole bonds formed in sulfide solution

We report on microdisk lasers based on GaInNAs(Sb)/GaAs(N) quantum well active region. Their characteristics were studied under electrical and optical pumping. Small-sized microdisks (minimal diameter 2.3 μm) with unprotected sidewalls show lasing only at temperatures below 220 K. Sulfide passivation followed by SiNx encapsulation allowed us achieving room temperature lasing at 1270 nm in 3 μm GaInNAs/GaAs microdisk and at 1550 nm in 2.3 μm GaInNAsSb/GaAsN microdisk under optical pumping. Injection microdisk with a diameter of 31 μm based on three GaInNAs/GaAs quantum wells and fabricated without passivation show lasing up to 170 K with a characteristic temperature of T0 = 60 K.

T. V. Lvova

Papers

Nitride Surface Passivation of GaAs Nanowires: Impact on Surface State Density

Wet sulfur passivation of GaSb(1 0 0) surface for optoelectronic applications

InP(1 0 0) surface passivation with aqueous sodium sulfide solution

Modification of the p-GaP(001) work function by surface dipole bonds formed in sulfide solution

Microdisk lasers based on GaInNAs(Sb)/GaAs(N) quantum wells