Showing papers on "Photoconductivity published in 2011"

Electron Mobility and Injection Dynamics in Mesoporous ZnO, SnO2, and TiO2 Films Used in Dye-Sensitized Solar Cells

Abstract: High-performance dye-sensitized solar cells are usually fabricated using nanostructured TiO2 as a thin-film electron-collecting material. However, alternative metal-oxides are currently being explored that may offer advantages through ease of processing, higher electron mobility, or interface band energetics. We present here a comparative study of electron mobility and injection dynamics in thin films of TiO2, ZnO, and SnO2 nanoparticles sensitized with Z907 ruthenium dye. Using time-resolved terahertz photoconductivity measurements, we show that, for ZnO and SnO2 nanoporous films, electron injection from the sensitizer has substantial slow components lasting over tens to hundreds of picoseconds, while for TiO2, the process is predominantly concluded within a few picoseconds. These results correlate well with the overall electron injection efficiencies we determine from photovoltaic cells fabricated from identical nanoporous films, suggesting that such slow components limit the overall photocurrent genera...

Band-like transport, high electron mobility and high photoconductivity in all-inorganic nanocrystal arrays

Abstract: Flexible, thin-film electronic and optoelectronic devices typically involve a trade-off between performance and fabrication cost. For example, solution-based deposition allows semiconductors to be patterned onto large-area substrates to make solar cells and displays, but the electron mobility in solution-deposited semiconductor layers is much lower than in semiconductors grown at high temperatures from the gas phase. Here, we report band-like electron transport in arrays of colloidal cadmium selenide nanocrystals capped with the molecular metal chalcogenide complex In(2)Se(4)(2-), and measure electron mobilities as high as 16 cm(2) V(-1) s(-1), which is about an order of magnitude higher than in the best solution-processed organic and nanocrystal devices so far. We also use CdSe/CdS core-shell nanoparticles with In(2)Se(4)(2-) ligands to build photodetectors with normalized detectivity D* > 1 × 10(13) Jones (I Jones = 1 cm Hz(1/2) W(-1)), which is a record for II-VI nanocrystals. Our approach does not require high processing temperatures, and can be extended to different nanocrystals and inorganic surface ligands.

Plasmonic dye-sensitized solar cells using core-shell metal-insulator nanoparticles.

Abstract: We present an investigation into incorporating core−shell Au−SiO2 nanoparticles into dye-sensitized solar cells. We demonstrate plasmon-enhanced light absorption, photocurrent, and efficiency for both iodide/triiodide electrolyte based and solid-state dye-sensitized solar cells. Our spectroscopic investigation indicates that plasmon-enhanced photocarrier generation competes well with plasmons oscillation damping with in the first tens of femtoseconds following light absorption.

Synthesis of Metallophthalocyanine Covalent Organic Frameworks That Exhibit High Carrier Mobility and Photoconductivity

Abstract: Covalent organic frameworks (COFs) are a new class of porous architectures that allow the integration of organic units with atomic precision into long-range-ordered twoand three-dimensional structures. 2] From a synthetic point of view, COFs are intriguing as they allow a new degree of control of porosity, composition, and component positions. As high-surface-area materials that link elements of low atomic mass by covalent bonds, COFs exhibit considerable potential for gas adsorption applications. As the first report on COFs in 2005, several families of COFs have been reported. However, the construction of COFs has to date been limited to certain monomers, and the lack of suitable procedures that utilize other units has impeded further advances in this emerging field. To advance this field it is therefore important to extend the limited number of synthetic methods and monomer units available. Phthalocyanines are large, planar p-electronic macrocycles with broad absorption profiles that could serve as intriguing units in the construction of porous frameworks. Crystalline phthalocyanine metal–organic frameworks have been shown to be useful in applications such as gas adsorption owing to their extended porous structures. However, phthalocyanine-based porous covalent polymers are usually amorphous and disordered. The combination of phthalocyanine units into a well-defined covalent framework thus remains an undeveloped area of research, offering great potential for obtaining novel functionality depending on the particular alignment and stacking. The eclipsed stacking endows arene-based COFs with unique functionality, such as excimer emission, exciton migration, and photoresponse. Herein, we have developed a new phthalocyanine unit for the synthesis of nickel phthalocyanine-based COFs (NiPc COF; Scheme 1). The compound, based on (2,3,9,10,16,17,23,24octahydroxyphthalocyaninato)nickel(II), [(OH)8PcNi], which has four catechol pairs at peripheral phenyl rings of a phthalocyanine macrocycle. These 2D COFs provide preorganized conduction paths based on precise ordering of the phthalocyanine stack and are ideal for charge carrier transport. Herein we present the high-throughput synthesis and unique properties of the two-dimensional metallophthalocyanine-based COF. The NiPc COF was synthesized by the boronate esterification reaction of [(OH)8PcNi] and 1,4-benzenediboronic acid (BDBA) in dimethylacetamide (DMAc)/o-dichlorobenzene under solvothermal conditions (Scheme 1a). [(OH)8PcNi] has low solubility in common organic solvents owing to its large p system and highly planar structure; typical procedures for the esterification reaction does not lead to the formation of desirable crystalline COFs. With reference to a well-established solvent combination (mesitylene/dioxane) for the synthesis of boronate-linked 2D COFs, 4] we investigated the reaction using different pairs of aromatic solvents (mesitylene, toluene, and o-dichlorobenzene) with hydrophilic solvents (dioxane, dimethylformamide (DMF), and dimethylacetamide (DMAc)). Combinations and results for the esterification reaction are summarized in the Supporting Information, Figure S1. The optimal combination for the preparation of the COFs was found to be o-dichlorobenzene and DMAc. Furthermore, the ratio of o-dichlorobenzene to DMAc was varied from 1:1 to 1:9 (vol/vol) to investigate the effect on crystallinity, as monitored by powder X-ray diffraction (PXRD) measurements. A mixture of o-dichlorobenzene/DMAc (1:2 vol/vol) gave the best result, as indicated by the intensity of the PXRD signals (Supporting Information, Figure S1). The NiPc COF was synthesized as a dark green powder in 90 % yield. It is notable that this method gives a yield that is much higher than the previously reported value (48%). Fourier-transform infrared (FTIR) spectra of the NiPc COF exhibited characteristic bands that are due to the boronate ester at 1053, 1089, 1291, and 1347 cm , and a typical C=N stretch at 1480 cm 1 for the phthalocyanine units (Supporting Information, Figure S2, Table S1). Solid-state H–C CP/MAS NMR spectroscopy using a 920 MHz H NMR spectrometer at a MAS rate of 15 kHz and a [*] X. Ding, Dr. J. Guo, X. Feng, Dr. P. Maitarad, Prof. Dr. D. Jiang Department of Materials Molecular Science Institute for Molecular Science 5-1 Higashiyama, Myodaiji, Okazaki 444-8787 (Japan) Fax: (+ 81)564-59-5520 E-mail: jiang@ims.ac.jp

Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films

Abstract: Colloidal semiconductor nanocrystals have attracted significant interest for applications in solution-processable devices such as light-emitting diodes and solar cells. However, a poor understanding of charge transport in nanocrystal assemblies, specifically the relation between electrical conductance in dark and under light illumination, hinders their technological applicability. Here we simultaneously address the issues of 'dark' transport and photoconductivity in films of PbS nanocrystals, by incorporating them into optical field-effect transistors in which the channel conductance is controlled by both gate voltage and incident radiation. Spectrally resolved photoresponses of these devices reveal a weakly conductive mid-gap band that is responsible for charge transport in dark. The mechanism for conductance, however, changes under illumination when it becomes dominated by band-edge quantized states. In this case, the mid-gap band still has an important role as its occupancy (tuned by the gate voltage) controls the dynamics of band-edge charges.

Transient photoconductivity in polymer bulk heterojunction solar cells: Competition between sweep-out and recombination

Abstract: Transient photoconductivity measurements carried out on bulk heterojunction (BHJ) solar cells demonstrate the competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination. The transient photoconductance data imply the existence of a well-defined internal field; carrier sweep-out is proportional to the magnitude of the internal field and limited by the carrier mobility. At external voltages near open circuit where the internal field approaches zero, the photocurrent decays because of the recombination of photogenerated mobile carriers. Mobility and recombination lifetimes are evaluated for carriers in poly[3-hexylthiophene] (P3HT): [6,6]-phenyl-C61 -butyric acid methyl ester (PC60 BM) and poly[N -9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT): [6,6]-phenyl-C71 -butyric acid methyl ester (PC71 BM) solar cells.

Deep-ultraviolet solar-blind photoconductivity of individual gallium oxide nanobelts.

Abstract: We designed solar-blind deep-ultraviolet semiconductor photodetectors using individual Ga2O3 nanobelts. The photoconductive behavior was systematically studied. The photodetectors demonstrate high selectivity towards 250 nm light, fast response times of less than 0.3 s, and a large photocurrent to dark current ratio of up to 4 orders of magnitude. The photoresponse parameters such as photocurrent, response time, and quantum efficiency depend strongly on the intensity of light, the detector environment, and the nanobelt size. The photoresponse mechanism was discussed, which was mainly attributed to the band bending, surface traps, and distribution of traps in the bandgap. Present Ga2O3 nanobelts can be exploited for future applications in photo sensing, light-emitting diodes, and optical switches.

Deep-ultraviolet solar-blind photoconductivity of individual gallium oxide nanobelts

Abstract: We designed solar-blind deep-ultraviolet semiconductor photodetectors using individual Ga2O3 nanobelts. The photoconductive behavior was systematically studied. The photodetectors demonstrate high selectivity towards 250 nm light, fast response times of less than 0.3 s, and a large photocurrent to dark current ratio of up to 4 orders of magnitude. The photoresponse parameters such as photocurrent, response time, and quantum efficiency depend strongly on the intensity of light, the detector environment, and the nanobelt size. The photoresponse mechanism was discussed, which was mainly attributed to the band bending, surface traps, and distribution of traps in the bandgap. Present Ga2O3 nanobelts can be exploited for future applications in photo sensing, light-emitting diodes, and optical switches.

Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires

Abstract: Photoconductivity is studied in individual ZnO nanowires. Under ultraviolet (UV) illumination, the induced photocurrents are observed to persist both in air and in vacuum. Their dependence on UV intensity in air is explained by means of photoinduced surface depletion depth decrease caused by oxygen desorption induced by photogenerated holes. The observed photoresponse is much greater in vacuum and proceeds beyond the air photoresponse at a much slower rate of increase. After reaching a maximum, it typically persists indefinitely, as long as good vacuum is maintained. Once vacuum is broken and air is let in, the photocurrent quickly decays down to the typical air-photoresponse values. The extra photoconductivity in vacuum is explained by desorption of adsorbed surface oxygen which is readily pumped out, followed by a further slower desorption of lattice oxygen, resulting in a Zn-rich surface of increased conductivity. The adsorption-desorption balance is fully recovered after the ZnO surface is exposed to air, which suggests that under UV illumination, the ZnO surface is actively "breathing" oxygen, a process that is further enhanced in nanowires by their high surface to volume ratio.

Infrared Emitting and Photoconducting Colloidal Silver Chalcogenide Nanocrystal Quantum Dots from a Silylamide-Promoted Synthesis

Abstract: Here, we present a hot injection synthesis of colloidal Ag chalcogenide nanocrystals (Ag2Se, Ag2Te, and Ag2S) that resulted in exceptionally small nanocrystal sizes in the range between 2 and 4 nm. Ag chalcogenide nanocrystals exhibit band gap energies within the near-infrared spectral region, making these materials promising as environmentally benign alternatives to established infrared active nanocrystals containing toxic metals such as Hg, Cd, and Pb. We present Ag2Se nanocrystals in detail, giving size-tunable luminescence with quantum yields above 1.7%. The luminescence, with a decay time on the order of 130 ns, was shown to improve due to the growth of a monolayer thick ZnSe shell. Photoconductivity with a quantum efficiency of 27% was achieved by blending the Ag2Se nanocrystals with a soluble fullerene derivative. The co-injection of lithium silylamide was found to be crucial to the synthesis of Ag chalcogenide nanocrystals, which drastically increased their nucleation rate even at relatively low g...

Photoconductivity measurements of the electronic structure of organic solar cells

Abstract: Experimental and theoretical studies of the electronic structure of bulk heterojunction (BHJ) organic solar cells are reported. The photoconductivity spectral response of the solar cells has a weak absorption band extending from the band-gap energy down to 1 eV due to charge-transfer optical excitation at the interface between the polymer and the fullerene. The low-energy absorption indicates an exponential band tail of localized states and an absorption model based on the one-electron joint density of electronic states accounts for the data. Transient photoconductivity measurements of the carrier mobility exhibit a temperature-dependent carrier dispersion. Data analysis for the particular case of transport in the BHJ structure is developed. A multiple trapping model of the dispersive transport is consistent with localized band tail states having a comparable density-of-states distribution to those observed by optical absorption. Theoretical calculations of the density of states including disorder in the \ensuremath{\pi}-\ensuremath{\pi} spacing of the polymer chains also shows exponential band tailing. A density-of-states model is developed from the data and is discussed.

Effect of top electrodes on photovoltaic properties of polycrystalline BiFeO3 based thin film capacitors.

Abstract: We investigated capacitors based on polycrystalline narrow-band-gap BiFeO(3) (BFO) thin films with different top electrodes. The photovoltaic response for the capacitor with a Sn-doped In(2)O(3) (ITO) top electrode is about 25 times higher than that with a Au top electrode, which indicates that the electrode plays a key role in determining the photovoltaic response of ferroelectric thin film capacitors, as simulated by Qin et al (2009 Appl. Phys. Lett. 95 22912). The light-to-electricity photovoltaic efficiency for the ITO/polycrystalline BFO/Pt capacitor can reach 0.125%. Furthermore, under incident light of 450 µW cm(-2) and zero bias, the corresponding photocurrent varies from 0.2 to 200 pA, that is, almost a 1000-fold photoconductivity enhancement. Our experiments suggest that polycrystalline BFO films are promising materials for application in photo-sensitive and energy-related devices.

Dark Carriers, Trapping, and Activation Control of Carrier Recombination in Neat P3HT and P3HT:PCBM Blends

Abstract: Using flash photolysis, time-resolved microwave conductivity we report the sub-200 ns photoconductivity transients for neat poly(3-hexylthiophene), P3HT, and four associated blends containing 1%, 5%, 20%, and 50%, by weight, of the soluble fullerene, [6,6]-phenyl-c61-butyric acid methyl ester, PCBM. We propose a detailed kinetic scheme that when solved numerically is consistent with all the data recorded as a function of excitation density and that describes the fate of mobile and trapped carriers in the system. In the neat polymer, mobile holes are the only contributor to the photoconductance transients, which decay according to first-order kinetics at all light intensities due to the presence of a large concentration of dark carriers present in the polymer. The signal decays with a characteristic rate constant (∼1 × 107 s–1) that describes the re-equilibration of trapped and mobile holes. In all four blends, the microwave absorption contains a significant contribution due to electrons in the PCBM cluste...

Solar-blind photodiodes composed of a Au Schottky contact and a β-Ga2O3 single crystal with a high resistivity cap layer

Abstract: We fabricated Ga2O3 photodiodes composed of a Au Schottky contact and a β-Ga2O3 single-crystal substrate with a sol–gel prepared high resistivity cap layer. The photodiodes with the cap layer showed solar-blind photosensitivity under both forward and reverse biases in contrast to conventional Schottky photodiodes. Finally, we proposed energy band diagram of the i-n junction to determine the photodetection mechanism of our photodiodes. The photoconductive device model explained the high responsivity of over 1 A/W at forward bias. In this model, the cap layer behaves like a photoconductor, and the substrate behaves like an electrode that replenishes electrons.

Solution processed reduced graphene oxide ultraviolet detector

Abstract: Electronic properties of graphene have been studied more extensively than its photonic applications, in spite of its exciting optical properties. Recent results on solar cells, light emitting diodes and photodetectors show its true potential in photonics and optoelectronics. Here, we have explored the use of reduced graphene oxide as a candidate for solution processed ultraviolet photodetectors. UV detection is demonstrated by reduced graphene oxide in terms of time resolved photocurrent as well as photoresponse. The responsivity of the detectors is found to be 0.12 A/W with an external quantum efficiency of 40%. (C) 2011 American Institute of Physics. [doi:10.1063/1.3640222]

Extended infrared photoresponse and gain in chalcogen-supersaturated silicon photodiodes

Abstract: Highly supersaturated solid solutions of selenium or sulfur in silicon were formed by ion implantation followed by nanosecond pulsed laser melting. n+p photodiodes fabricated from these materials exhibit gain (external quantum efficiency >3000%) at 12 V of reverse bias and substantial optoelectronic response to light of wavelengths as long as 1250 nm. The amount of gain and the strength of the extended response both decrease with decreasing magnitude of bias voltage, but >100% external quantum efficiency is observed even at 2 V of reverse bias. The behavior is inconsistent with our expectations for avalanche gain or photoconductive gain.

WO3 nanowires on carbon papers: electronic transport, improved ultraviolet-light photodetectors and excellent field emitters

Abstract: Single-crystalline WO3 nanowires were synthesized on carbon papers through a chemical vapor deposition process without using any catalysts. WO3 nanowire field-effect transistors (FETs) were constructed to discuss the mechanism of electronic transport based on a thermal-activation model and a displacive transition. Photoconductive measurements showed that individual WO3 nanowire photodetector was sensitive to the ultraviolet light, and the photoresponse was further improved using WO3 nanowires on carbon papers, demonstrating significantly shortened response and decay times, and enhanced stability. Field-emission measurements showed that WO3 nanowires were excellent field-emitters: an ultralow turn-on field of 1.8 V μm−1 and a threshold field of 3.3 V μm−1, and a high field-enhancement factor of 6.9 × 103. These results indicate that present unique WO3 nanowires on carbon papers are promising candidates for constructing high-performance electronic and optoelectronic devices.

Single-crystalline ZnTe nanowires for application as high-performance green/ultraviolet photodetector.

Abstract: Single-crystalline ZnTe nanowires were prepared by a simple vapor transport and deposition method. Photodetectors of individual ZnTe nanowires were fabricated to study photoconductivity of the nanowires. It was observed the nanowire photodetectors show the highest visible-light photoconductive gains among all reported photodetectors based on 1D nanostructure semiconductors, including CdS, CdSe, ZnSe, etc. The high photosensitivity and relatively fast response speed are attributable to the high crystal quality of the ZnTe nanowires. These results reveal that such single-crystalline ZnTe nanowires are excellent candidates for optoelectronic applications.

High gain ultraviolet photodetectors based on AlGaN/GaN heterostructures for optical switching

Abstract: We report on the optoelectronic properties of Al0.25Ga0.75N/GaN-based ultraviolet (UV) photodetectors for the application as a high current, high gain optical switch. Due to an internal gain mechanism combined with the high conductivity of the two-dimensional electron gas at the heterostructure interface, photocurrents in the milliampere-range were obtained with UV illumination. By employing a mesa structure design with meander geometry very low dark currents below 50 nA up to a bias voltage of 100 V were achieved. Optical switching with an on/off-current-ratio of five orders of magnitude was demonstrated. The response time was determined to be 6 ms and persistent photoconductivity was observed. The photodetector is visible-blind with a cut-off wavelength of 365 nm according to the band gap energy of the GaN absorption layer. A high responsivity with a maximum of 70 A/mW at 312 nm and 100 V bias voltage was demonstrated.

ZnO homojunction photodiodes based on Sb-doped p-type nanowire array and n-type film for ultraviolet detection

Abstract: ZnO p-n homojunctions based on Sb-doped p-type nanowire array and n-type film were grown by combining chemical vapor deposition (for nanowires) with molecular-beam epitaxy (for film). Indium tin oxide and Ti/Au were used as contacts to the ZnO nanowires and film, respectively. Characteristics of field-effect transistors using ZnO nanowires as channels indicate p-type conductivity of the nanowires. Electron beam induced current profiling confirmed the existence of ZnO p-n homojunction. Rectifying I-V characteristic showed a turn-on voltage of around 3 V. Very good response to ultraviolet light illumination was observed from photocurrent measurements.

Self-Cleaning Flexible Infrared Nanosensor Based on Carbon Nanoparticles

Abstract: Highly flexible, robust, and sensitive infrared nanosensors were fabricated based on carbon nanoparticles that were synthesized through a simple and low-cost flame method. The infrared nanosensor devices showed sharp infrared photoresponse with a response time of ∼68 ms and a maximum photocurrent change of ∼52.9%. The devices showed a superhydrophobic property with a contact angle larger than 150° and a sliding angle of ∼4°. The mechanism for the enhanced infrared photoresponse from carbon nanoparticles is discussed.

Highly sensitive fast-response UV photodetectors based on epitaxial TiO2 films

Abstract: Epitaxial TiO2 thin films were fabricated on LaAlO3 single crystal substrates by RF magnetron sputtering. Ag electrodes were then evaporated on the TiO2 thin films to form metal–semiconductor–metal photoconductive detectors. The TiO2 photodetector exhibited a maximum photoresponse of 3.63 A W−1 at 310 nm with a sharp cutoff wavelength at 380 nm. The ultraviolet (UV)–visible response rejection ratio (R310 nm/R390 nm) was about three orders of magnitude. The photocurrent response of the detectors scaled linearly with the applied bias and the incident light intensity. The dark current was only 0.14 nA at 10 V bias. A transient photovoltage with a rise time of ~8 ns and a full-width at half-maximum of ~90 ns was observed when the photodetector was under the irradiation of a 308 nm XeCl laser with 25 ns duration. The excellent performances of high responsivity and ultrahigh response speed suggest that the presented TiO2 detectors have promising potential in UV photodetection.

Surface induced negative photoconductivity in p-type ZnSe : Bi nanowires and their nano-optoelectronic applications

Abstract: Single-crystal p-type ZnSe nanowires (NWs) with zinc blende structure and [21–1] growth direction were synthesized by using bismuth (Bi) as dopant via a thermal co-evaporation method. The ZnSe:Bi NWs showed evident p-type conductivity with hole concentration up to 4.1 × 1018 cm−3 after the acceptor activation. Negative photoconductivity was first investigated in the p-ZnSe NWs, i.e., the conductivity of the NWs under light was dramatically lower than that in the dark. Surface effects arising from oxygen absorption and photo-desorption were suggested to be responsible for this. By using Al as the Schottky gate, high-performance nano-metal-semiconductor field-effect transistors (nanoMESFETs) were constructed, and measurements on the Al/p-ZnSe NW Schottky diode also revealed the bias-dependent photoresponse. It is expected that the p-type ZnSe:Bi NWs will have great potential in nano-optoelectronic applications.

White emission of lithium ytterbium tetraphosphate nanocrystals.

Abstract: An efficient anti-Stokes white broadband emission induced by 976 nm laser diode in lithium ytterbium tetraphosphate (LiYbP4O12) nanocrystals was investigated. The emission occurs at room temperature and atmospheric pressure. Its intensity demonstrates an evident threshold dependence on the temperature and excitation density characteristic to avalanche process. The white emission is accompanied by very efficient photoconductivity characterized by microampere photocurrent which increases with the fourth order of applied incident light power (~P4). We show that this emission is critically dependent on temperature and increases significantly in vacuum. It is concluded that the anti-Stokes white emission is associated with theYb3+- CT luminescence.

High-Performance Near-IR Photodiodes: A Novel Chemistry-Based Approach to Ge and Ge–Sn Devices Integrated on Silicon

Abstract: Ge/Si heterostructure diodes based on n++Si(100)/i-Ge/p-Ge and p++Si(100)/i-Ge/n-Ge stacks and intrinsic region thickness of ~350 and ~900 nm, respectively, were fabricated using a specially developed synthesis protocol that allows unprecedented control of film microstructure, morphology, and purity at complementary metal-oxide-semiconductor compatible conditions. From a growth and doping perspective, a main advantage of our inherently low-temperature (390°C) soft-chemistry approach is that all high-energy processing steps are circumvented. Current-voltage measurements of circular mesas (60-250 μm in diameter) show dark current densities as low as 6 ×10-3 A/cm2 at -1 V bias, which is clearly improved over devices fabricated under low thermal budgets using traditional Ge deposition techniques. Spectral photocurrent measurements indicate external quantum efficiencies between 30 and 60% of the maximum theoretical value at zero bias, and approaching full collection efficiency at high reverse biases. The above Ge devices are compared to analogous low-temperature-grown (350°C) Ge0.98Sn0.02 diodes. The latter display much higher dark currents but also higher collection efficiencies close to 70% at zero bias. Moreover, the quantum efficiency of these Ge0.98Sn0.02 diodes remains strong at wavelengths longer than 1550 nm out to 1750 nm due to the reduced band gap of the alloy relative to Ge.

Spatially Resolved Plasmonically Enhanced Photocurrent from Au Nanoparticles on a Si Nanowire

Abstract: Semiconducting nanowires have been demonstrated as promising light-harvesting units with enhanced absorption compared to bulk films of equivalent volume. However, for small diameter nanowires, the ultrahigh aspect ratio constrains the absorption to be polarization selective by responding primarily to the transverse magnetic (TM) light. While this effect is useful for polarization-sensitive optoelectronic devices, practical light-harvesting applications demand efficient light absorption in both TM and transverse electric (TE) light. In this study, we engineer the polarization sensitivity and the charge carrier generation in a 50 nm Si nanowire by decorating the surface with plasmonic Au nanoparticles. Using scanning photocurrent microscopy (SPCM) with a tunable wavelength laser, we spatially and spectrally resolve the local enhancement in the TE photocurrent resulting from the plasmonic near-field response of individual nanoparticles and the broad-band enhancement due to surface-enhanced absorption. These results provide guidance to the development and the optimization of nanowire-nanoparticle light-harvesting systems.

Single-wire photodetectors based on InGaN/GaN radial quantum wells in GaN wires grown by catalyst-free metal-organic vapor phase epitaxy

Abstract: We present a letter on single-wire photodetectors based on radial n-i-n multiquantum well (QW) junctions The devices are realized from GaN wires grown by catalyst-free metalorganic vapor phase epitaxy coated at their top by five nonpolar In016Ga084N/GaN undoped radial QWs, and are sensitive to light with energy E>26 eV Their photoconductive gain is as high as 2×103 The scanning photocurrent microscopy maps evidence that the detector response is localized at the extremity containing the QWs for both below (at λ=488 nm) and above GaN band gap (at λ=244 nm) excitation This confirms that the device operates as a radial n-i-n junction