We report the fabrication of multi-branched TiO2/Si nanowire arrays and their application as the photoelectrochemical electrode for solar water splitting. The multi-branched TiO2/Si nanowire arrays improved the photoelectrochemical (PEC) performance compared to TiO2 thin film-coated Si nanowire arrays because of its enlarged surface area for electrochemical reactions and enhanced charge transfer kinetics. Wavelength dependent photocurrent response of the multi-branched nanowire array indicates a strong response in the ultraviolet region (<400 nm), while a negligible photocurrent under visible illumination primarily caused by the high overpotential loss of n-Si photoanode and energy band configuration of the TiO2/Si heterojunction. The working mechanism based on recombination at this heterogeneous n-n junction was further vieified. This study provides insights on the intrinsic problem and potential capabilities/optimizations of nanoscale heterogeneous devices for renewable energy applications.
Abstract: We report the fabrication of high performance broadband polymer photodetector based on small bandgap conjugated polymers with an inverted device structure, where electrons and holes are collected on ITO and metal contact with high working functions. High quality wide bandgap vertically aligned ZnO nanowire array offers an enhanced surface area and is used as the cathode in this device for effectively extracting electrons and blocking holes from the active polymer layer. The room temperature detectivity of polymer PDs with such an inverted device structure is greater than 1010 Jones with the spectral response from 400 nm to 1450 nm. Our results define a promising pathway for fabrication of high sensitivity polymer PDs with an inverted device structure using ZnO nanowire array cathode buffer layer for a wide range of applications.
Abstract: We report a nickel oxide (NiOx) thin film, from a cost-effective sol-gel process, coated n-type silicon (n-Si) as a photoanode for efficient photo-oxidation of water under neutral pH condition. The NiOx thin film has three functions: (i) serves as a protection layer to improve the chemical stability of Si photoelectrode, (ii) acts as an oxygen evolution catalyst, and (iii) provides junction photovoltage to further reduce overpotential. The oxygen evolution onset potential is reduced to below the thermodynamic water oxidation level and oxygen evolution was observed at low overpotentials. Our results demonstrate the fabrication of robust photoelectrodes from low-cost NiOx and Si, which enable a practical solar water oxidation with high efficiency.
Abstract: We report a systematic study of Si|ZnO and Si|ZnO|metal photocathodes for effective photoelectrochemical cells and hydrogen generation. Both ZnO nanocrystalline thin films and vertical nanowire arrays were studied. Si|ZnO electrodes showed increased cathodic photocurrents due to improved charge separation by the formation of a p/n junction, and Si|ZnO:Al (n+-ZnO) and Si|ZnO(N2) (thin films prepared in N2/Ar gas) lead to a further increase in cathodic photocurrents. Si|ZnONW (nanowire array) photocathodes dramatically increased the photocurrents and thus photoelectrochemical conversion efficiency due to the enhanced light absorption and enlarged surface area. The ZnO film thickness and ZnO nanowire length were important to the enhancements. A thin metal coating on ZnO showed increased photocurrent due to a catalyzed hydrogen evolution reaction and Ni metal showed comparable catalytic activities to those of Pt and Pd. Moreover, photoelectrochemical instability of Si|ZnO electrodes was minimized by metal co-catalysts. Our results indicate that the metal and ZnO on p-type Si serve as co-catalysts for photoelectrochemical water splitting, which can provide a possible low-cost and scalable method to fabricate high efficiency photocathodes for practical applications in clean solar energy harvesting.
Abstract: We report the fabrication of a three dimensional branched ZnO/Si heterojunction nanowire array by a two-step, wafer-scale, low-cost, solution etching/growth method and its use as photoelectrode in a photoelectrochemical cell for high efficiency solar powered water splitting. Specifically, we
demonstrate that the branched nanowire heterojunction photoelectrode offers improved light absorption, increased photocurrent generation due to the effective charge separation in Si nanowire backbones and ZnO nanowire branching, and enhanced gas evolution kinetics because of the dramatically increased surface area and decreased radius of curvature. The branching nanowire heterostructures offer direct functional integration of different materials for high efficiency water photoelectrolysis and scalable photoelectrodes for clean hydrogen fuel generation. issue cover (most read articles) (News highlight)
Abstract: Single crystalline vertical ZnO nanorods were grown in a one hole–one rod configuration using a hydrothermal method with a patterned polymer template generated by nanoimprint lithography, allowing precise control over the position and density of the ZnO nanorods. An 8 × 8 ZnO nanorod-based ultraviolet photodetector array is demonstrated, in which a well-confined number of ZnO nanorods are sandwiched between crossbar-type platinum and indium tin oxide electrodes (e.g. 16 nanorods in a 2 × 2 µm2 area). A high photocurrent/dark current ratio of 3 × 103 at a reverse bias of 1.5 V under UV illumination at room temperature, a responsivity of 4381.4 A W − 1 at 365 nm, and an ultraviolet-to-visible rejection ratio of 83 are obtained and maintained, irrespective of pixel size. A uniform photoresponse is achieved in each of the pixels, indicating the scalability with this technique for fabricating an integrated UV photodetector array circuit.
Abstract: We investigated the enhanced photoresponse of ZnO nanowire transistors that was introduced with surface-roughness-induced traps by a simple chemical treatment with isopropyl alcohol (IPA). The enhanced photoresponse of IPA-treated ZnO nanowire devices is attributed to an increase in adsorbed oxygen on IPA-induced surface traps. The results of this study revealed that IPA-treated ZnO nanowire devices displayed higher photocurrent gains and faster photoswitching speed than transistors containing unmodified ZnO nanowires. Thus, chemical treatment with IPA can be a useful method for improving the photoresponse of ZnO nanowire devices.
Abstract:In this work, we have successfully synthesis ZnO crystal thin film with a high quality from hydrothermal reaction on sapphire substrate. Growth mechanism is clarified based on extensive XTEM study. In addition, electrical and optical properties of the crystal thin film can be controlled by impurities are characterized. The synthesis technique provides fairly high quality and cost-effective substrate for optoelectronic and renewable energy applications.
Abstract:There have been many recent developments in compound semiconductor nanowire photovoltaic devices. Of these, advances in nanowire synthesis and performance enable nanowires to be implemented for efficient and low cost solar energy harvesting devices. On the other hand, many challenges in device fabrication must be resolved in order for nanowires to assure a role at the forefront of solar cell technology.
Abstract: This communication reports a low-cost solution fabrication of wafer-scale ZnO/Si branched nanowire heterostructures and their high photodetection sensitivity of ON/OFF ratio larger than 250 and a peak photo responsivity of 12.8 mA/W at 900 nm. This reported unique 3D branched nanowire structures offer generic approach for the integration of new functional materials for photodetection and photovoltaic applications.
46C. Soci, A. Zhang, X. Bao, H. Kim, Y. Lo and D. Wang, “Nanowire photodetector”, Journal of Nanoscience and Nanotechnology 10, 1430 2010. (pdf)
Abstract: The use of nanowires and nanowire structures as photodetectors is an emerging research topic. Despite the large amount of reports on nanowire photoresponse that appeared in the literature over the last decade, the mechanism leading to high photosensitivity and photoconductive gain in high aspect ratio nanostructures has been elucidated only recently. Novel device architectures integrated in single nanowire devices are also being actively studied and developed. In this article, the general nanowire photodetector concepts are reviewed, together with a detailed description of the physical phenomena occurring in nanowire photoconductors and phototransistors, with some examples from recent experimental results obtained in our groups. An outlook on future directions toward the use of semiconductor nanowire photoconductors as intrachip interconnects, single-photon detectors, and image sensors, is also given.
Abstract:
Catalyst-free, direct heteroepitaxial growth of vertical InAs nanowires on Si(111) substrates was accomplished over a large area by metal−organic chemical vapor deposition. Nanowires showed very uniform diameters and a zinc blende crystal structure. The heterojunctions formed at the interface between the n-type InAs nanowires and the p-type Si substrate were exploited to fabricate vertical array photodiode devices which showed an excellent rectification ratio and low reverse leakage current. Temperature-dependent current transport across the heterojunctions was studied theoretically and experimentally in the dark and under AM 1.5 illumination.
Abstract:
New materials and device concepts are in great demand for continual (opto)electronic device scaling and performance enhancement. Arsenide III-V semiconductor nanowires promise novel device architectures and superior (opto)electronic properties. Recent insights into the growth and optimal control over the InAs and GaAs nanowire morphology and distinguished key physical aspects in their growth are discussed. Direct correlation of individual nanowire crystal structure with their electronic transport properties is also presented.
Abstract:
We demonstrate the ability to precisely control the alignment and placement of large numbers of InAs nanowires from solution onto very narrow, prepatterned electrodes using dielectrophoresis. An understanding of dielectrophoretic behavior associated with such electrode geometries is essential to development of approaches for assembly of intricate nanowire systems. The influence of signal frequency and electrode design on nanowire manipulation and placement is examined. Signal frequencies in the range of 10 MHz are found to yield high percentages of aligned nanowires on electrodes with dimensions similar to that of the nanowire. Strategies for further improvement of nanowire alignment are suggested and analyzed.
Abstract:
We report new fundamental insights into InAs nanowire (NW) nucleation and evolution on InAs (111)B surfaces using organometallic vapor phase epitaxy and present the first experimental demonstration of two distinct NW growth regimes, defined by the direction of substrate−NW adatom exchange, that lead to nonlinear growth rates. We show that the NW elongation rate and morphology in these two growth regimes are governed by the relative difference between the In adatom diffusion lengths on the growth substrate surface and on the NW sidewalls, resulting in strong growth rate dependence on the NW length. These results indicate that surface solid−phase diffusion of In adatoms is a key process in InAs NW growth, which is also supported by diameter-dependent growth rates. These developments enable rational growth of axial and radial NW heterostructures.
Abstract:
Here, direct correlation between the microstructure of InAs nanowires (NWs)
and their electronic transport behavior at room temperature is reported. Pure
zinc blende (ZB) InAs NWs grown on SiO2/Si substrates are characterized by
a rotational twin along their growth-direction axis while wurtzite (WZ) InAs
NWs grown on InAs (111)B substrates have numerous stacking faults
perpendicular to their growth-direction axis with small ZB segments. In
transport measurements on back-gate field-effect transistors (FETs)
fabricated from both types of NWs, significantly distinct subthreshold
characteristics are observed (Ion/Ioff2 for ZB NWs and 104 for WZ NWs)
despite only a slight difference in their transport coefficients. This difference is
attributed to spontaneous polarization charges at the WZ/ZB interfaces,
which suppress carrier accumulation at the NW surface, thus enabling full
depletion of the WZ NW FET channel. More>>>
Abstract: We have used scanning capacitance microscopy and spectroscopy to examine the effects of micron-scale metal contacts, typically present in nanowire-based electronic devices, on carrier modulation and electrostatic behavior in InAs semiconductor nanowires. We observe a pronounced dependence of scanning capacitance images and spectra on distance between the scanning capacitance probe tip and nanowire contact up to distances of 3–4 µm. Based on the comparison of these data with results of finite-element electromagnetic simulations, we interpret these results as a consequence of electrostatic screening of the tip-nanowire potential difference by the large metal contact. More>>>
Abstract:
Herein, we present a comprehensive method to extract the transport coefficients in NWs as functions of their diameter and vertical (gate) and lateral (drain) fields, thus enabling
consistent comparison due to their field dependence. This extraction technique extends earlier studies on the variation of transport properties in NWs as functions of their diameter, which was attributed to either 1) only changes in carrier concentration (Si NWs) or 2) only changes in carrier mobility (GaN NWs). One-dimensional (1D) Schro¨ dinger– Poisson self-consistent solutions for a material structure similar to that used in our experiments, but with different InAs thicknesses, was performed to validate our experimental observations and analysis.
Abstract:
Nanowire photodetectors are shown to function as phototransistors with high sensitivity. Due to small lateral dimensions, a nanowire detector can have low dark current while showing large phototransistive gain. Planar and vertical silicon nanowire photodetectors fabricated in a top-down approach using an etching process show a phototransistive gain above 35 000 at low light intensities. Simulations show that incident light can be waveguided into vertical nanowires resulting in up to 40 times greater external quantum efficiency above their physical fill factor. Vertical silicon nanowire phototransistors formed by etching are attractive for low light level detection and for integration with silicon electronics.
Abstract:
Vertical and electrically isolated InAs nanowires (NWs) are integrated with Si in a technique that bypasses structural defects and transport barriers at the Si–III–V NW interface. Smart-cut® technique is used to transfer a thin InAs layer onto SiO2/Si and is subsequently used for ordered organometallic vapor phase epitaxy of InAs NWs. The InAs layer in the regions between the InAs NWs is etched resulting in ordered, vertical, and electrically isolated InAs NW arrays. This transfer and fabrication technique enables heteroepitaxy of three dimensional III–V structures on Si and allows the realization of vertical devices with unprecedented control over their architectures.
Abstract:
We present detailed studies of the field dependent transport properties of InAs nanowire field-effect transistors. Transconductance dependence on both vertical and lateral fields is discussed. Velocity-field plots are constructed from a large set of output and transfer curves that show negative differential conductance behavior and marked mobility degradation at high injection fields. Two dimensional electrothermal simulations at current densities similar to those measured in the InAs NWFET devices indicate that a significant temperature rise occurs in the channel due to enhanced phonon scattering that leads to the observed mobility degradation. Scanning transmission electron microscopy measurements on devices operated at high current densities reveal arsenic vaporization and crystal deformation in the subject nanowires.
Abstract:
The epitaxial growth of GaAs nanowires (NWs) on GaAs(111)B substrates by metal−organic chemical vapor deposition has been systematically investigated as a function of relevant growth parameters, namely, temperature, arsine (AsH3) and trimethyl-gallium (TMGa) flow rates, growth time, and gold nanoparticle catalyst size. When growing in excess As conditions (V/III molar ratios greater than four), the NW growth rate is independent of AsH3 concentration, while it is linearly dependent on TMGa concentration, and it is thermally activated. The NW morphology is primarily affected by the growth temperature, with very uniform NWs growing at around 400 °C and severely tapered NWs growing above 500 °C. A simple phenomenological expression that allows prediction of the NW growth rate over a wide range of growth parameters has been derived. The growth rate dependence on the seed nanoparticle size has also been investigated, which reveals valuable information on the role of catalyst supersaturation and Ga surface diffusion in the growth mechanism. The NW growth rate is found to be almost independent of Au nanoparticle size down to diameters of 20 nm over a wide range of temperatures and TMGa and AsH3 molar flows. More>>>
Abstract:
Epitaxial growth of vertical GaAs nanowires on Si (111) substrates is demonstrated by metal−organic chemical vapor deposition via a vapor−liquid−solid growth mechanism. Systematic experiments indicate that substrate pretreatment, pregrowth alloying temperature, and growth temperature are all crucial to vertical epitaxial growth. Nanowire growth rate and morphology can be well controlled by the growth temperature, the metal−organic precursor molar fraction, and the molar V/III ratio. The as-grown GaAs nanowires have a predominantly zinc-blende crystal structure along a 111 direction. Crystallographic {111} stacking faults found perpendicular to the growth axis could be almost eliminated via growth at high V/III ratio and low temperature. Single nanowire field effect transistors based on unintentionally doped GaAs nanowires were fabricated and found to display a strong effect of surface states on their transport properties.
Abstract:
In this study, we examine nucleation and growth of InAs NWs from excess In with and without Au nanoparticles. Au nanoparticles on SiO2/Si substrates were found to facilitate
AsH3 pyrolysis[18] but not to be necessary to nucleate NWs, while excess In, supplied from either the input group III precursor source or the III–V substrate, was found to nucleate
InAs NWs. We show that due to the catalytic effect of Au nanoparticles on substrate decomposition, NW growth in a closed chemical vapor deposition (CVD) tube is possible
without any additional source other than the growth substrate itself.
Abstract:
We have studied the nucleation and growth of InAs nanowires (NWs) on SiO2/Si substrates by organometallic vapor-phase epitaxy (OMVPE). Through systematic characterization of InAs NW morphology as a function of V/III precursor ratio, precursor flow rates, growth temperature, growth time, and the presence/absence of Au nanoparticles, a number of significant insights into InAs NW growth using OMVPE have been developed. Specifically, we have found that (i) the growth of InAs NWs can be initiated from a single indium (In) droplet, (ii) Au nanoparticles (NPs) enhance group V precursor (AsH3) pyrolysis but are not necessary to nucleate growth, (iii) growth of InAs NWs on SiO2 substrates occurs in the kinetically limited vapor−liquid−solid (VLS) growth regime, (iv) InAs NWs on SiO2 films decompose at elevated temperatures even under significant AsH3 overpressure, and (v) the V/III ratio is the growth-rate-limiting factor in the VLS growth of the InAs nanowires. Many of these findings on InAs NW growth can be generalized to and provide very useful information for rational synthesis of other III−V compound semiconductor NWs.
Abstract:
It is shown that interface trap states have pronounced effects on carrier transport and parameter extraction from top-gated InAs nanowire field effect transistors (NWFETs). Due to slow surface state charging and discharging, the NWFET characteristics are time dependent with time constants as long as ~45 s. This is also manifested in a time-dependent extrinsic transconductance that severely affects carrier mobility and carrier density determination from conventional three-terminal current-voltage characteristics. Slow gate voltage sweep rates result in charge balance between carrier capture and emission from interface states and lead to reduced hysteresis in the transfer curves. The gate transconductance is thus increased and intrinsic NW transport parameters can be isolated. In the InAs NWFETs, a carrier mobility value of ~16 000 cm2/V s was obtained from the transfer curves at slow sweep rates, which is significantly higher than ~1000 cm2/V s obtained at fast sweep rates. A circuit model that takes into account the reduction in the extrinsic transconductance is used to estimate an interface state capacitance to be ~2 µF/cm−2, a significant value that can lead to underestimation of carrier mobility.
Abstract:
The authors have used scanning gate microscopy combined with numerical simulations to analyze local carrier and current modulation effects in InAs semiconductor nanowires grown by metal-organic chemical vapor deposition. Measurements of current flow in the nanowire as a function of probe tip position, at both high and low drain bias, reveal that carrier and current modulation is strongest when the probe tip is near the source and drain nanowire contacts, and decreases at greater tip-contact distances. The measured transconductance is approximately 80% greater near the source contact for high drain bias condition and 120% greater near the drain contact for low drain bias condition, respectively, than at the center of the nanowire. Numerical simulations for different tip positions relative to the metal contact confirm that carrier modulation should be stronger when the tip is closer to the source or drain contact than at the center of the wire, consistent with the experimental measurements.
Abstract:
We have studied the dependence of Au-assisted InAs nanowire (NW) growth on InAs(111)B substrates as a function of substrate temperature and input V/III precursor ratio using organometallic vapor-phase epitaxy. Temperature-dependent growth was observed within certain temperature windows that are highly dependent on input V/III ratios. This dependence was found to be a direct consequence of the drop in NW nucleation and growth rate with increasing V/III ratio at a constant growth temperature due to depletion of indium at the NW growth sites. The growth rate was found to be determined by the local V/III ratio, which is dependent on the input precursor flow rates, growth temperature, and substrate decomposition. These studies advance understanding of the key processes involved in III−V NW growth, support the general validity of the vapor−liquid−solid growth mechanism for III−V NWs, and improve rational control over their growth morphology.
Abstract:
Scanning gate microscopy, in which a conductive probe tip in an atomic force microscope is employed as a local, nanoscale top gate contact, has been used to characterize local carrier and current modulation effects in a 45 nm diameter InAs semiconductor nanowire grown by metal organic chemical vapor deposition. Measurement of current flow in the nanowire as a function of tip position reveals that for both positive and negative tip bias voltages, carrier and current modulation is strongest when the probe tip is near the source and drain nanowire contacts, reaching a global maximum approximately 100–200 nm distant from the source contact and a secondary maximum a similar distance from the drain contact and decreasing at greater tip-contact distances. This effect is explained, with verification by numerical simulation, as a consequence of the capacitance between the tip and the source and drain contacts as a function of tip location. Measurement of transconductance as a function of tip position reveals that the transconductance is approximately 80%–90% greater near the source contact than at the center of the nanowire.
Abstract:
The capacitive effects of interface trap states in top-gated InAs nanowire field effect transistors and their influence on the experimental extraction of transport parameters are discussed. Time resolved transfer characteristics exhibit transient behavior indicating surface state trapping and detrapping with long characteristic time constants of 45 s. Varying gate voltage sweep rate results in a time-dependent extrinsic transconductance; a reduced gate voltage sweep rate leads to a charge neutral interface, reduced interface state capacitance, higher measured transconductance, and minimal hysteresis. These results demonstrate that measurements with a charge neutralized or passivated surface are key to extract intrinsic nanowire transport parameters.
Abstract:
ZnO nanowire (NW) visible-blind UV photodetectors with internal photoconductive gain as high as G 108 have been fabricated and characterized. The photoconduction mechanism in these devices has been elucidated by means of time-resolved measurements spanning a wide temporal domain, from 10-9 to 102 s, revealing the coexistence of fast (τ 20 ns) and slow (τ 10 s) components of the carrier relaxation dynamics. The extremely high photoconductive gain is attributed to the presence of oxygen-related hole-trap states at the NW surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime, as evidenced by the sensitivity of the photocurrrent to ambient conditions. Surprisingly, this mechanism appears to be effective even at the shortest time scale investigated of t < 1 ns. Despite the slow relaxation time, the extremely high internal gain of ZnO NW photodetectors results in gain-bandwidth products (GB) higher than 10 GHz. The high gain and low power consumption of NW photodetectors promise a new generation of phototransistors for applications such as sensing, imaging, and intrachip optical interconnects.
Abstract:
We report, for the first time, the synthesis of the high-quality p-type ZnO NWs using a simple chemical vapor deposition method, where phosphorus pentoxide has been used as the dopant source. Single-crystal phosphorus doped ZnO NWs have their growth axis along the 001 direction and form perfect vertical arrays on a-sapphire. P-type doping was confirmed by photoluminescence measurements at various temperatures and by studying the electrical transport in single NWs field-effect transistors. Comparisons of the low-temperature PL of unintentionally doped ZnO (n-type), as-grown phosphorus-doped ZnO, and annealed phosphorus-doped ZnO NWs show clear differences related to the presence of intragap donor and acceptor states. The electrical transport measurements of phosphorus-doped NW FETs indicate a transition from n-type to p-type conduction upon annealing at high temperature, in good agreement with the PL results. The synthesis of p-type ZnO NWs enables novel complementary ZnO NW devices and opens up enormous opportunities for nanoscale electronics, optoelectronics, and medicines.
Single-crystal InAs nanowires (NWs) are synthesized using metal-organic chemical vapor deposition (MOCVD) and fabricated into NW field-effect transistors (NWFETs) on a SiO2/n+-Si substrate with a global n+-Si back-gate and sputtered SiOx/Au underlap top-gate. For top-gate NWFETs, we have developed a model that allows accurate estimation of characteristic NW parameters, including carrier field-effect mobility and carrier concentration by taking into account series and leakage resistances, interface state capacitance, and top-gate geometry. Both the back-gate and the top-gate NWFETs exhibit room-temperature field-effect mobility as high as 6580 cm2 V-1 s-1, which is the lower-bound value without interface-capacitance correction, and is the highest mobility reported to date in any semiconductor NW.
K. Sun, W. Wei, Y. Jing, Yong Ding, Zhong Lin Wang, and D. Wang “Crystalline ZnO Thin Film by Hydrothermal Growth ”, ChemCommun, 2011, 47, 7776 - 7778. (pdf)
