Search

Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells.

Chemical doping has been investigated as an alternative method of conventional ion implantation for two-dimensional materials. We herein report chemically doped multilayer molybdenum disulfide (MoS2) field effect transistors (FETs) through n-type channel doping, wherein triethanolamine (TEOA) is used as an n-type dopant. As a result of the TEOA doping process, the electrical performances of multilayer MoS2 FETs were enhanced at room temperature. Extracted field effect mobility was estimated to be ∼30 cm2 V−1 s−1 after the surface doping process, which is 10 times higher than that of the pristine device. Subthreshold swing and contact resistance were also improved after the TEOA doping process. The enhancement of the subthreshold swing was demonstrated by using an independent FET model. Furthermore, we found that the doping level can be effectively controlled by the heat treatment method. These results demonstrate a promising material system that is easily controlled with high performance, while elucidating the underlying mechanism of improved electrical properties by the doping effect in a multilayered scheme.

Device configurations that enable a unidirectional propagation of carriers in a semiconductor are fundamental components for electronic and optoelectronic applications. To realize such devices, however, it is generally required to have complex processes to make p-n or Schottky junctions. Here we report on a unidirectional propagation effect due to a self-induced compositional variation in GaAsSb nanowires (NWs). The individual GaAsSb NWs exhibit a highly reproducible rectifying behavior, where the rectifying direction is determined by the NW growth direction. Combining the results from confocal micro-Raman spectroscopy, electron microscopy, and electrical measurements, the origin of the rectifying behavior is found to be associated with a self-induced variation of the Sb and the carrier concentrations in the NW. To demonstrate the usefulness of these GaAsSb NWs for device applications, NW-based photodetectors and logic circuits have been made.

Semiconductor nanowire lasers can produce guided coherent light emission with miniaturized geometry, bringing about new possibilities for a variety of applications including nanophotonic circuits, optical sensing, and on-chip and chip-to-chip optical communications. Here, we report on the realization of single-mode and room-temperature lasing from 890 nm to 990 nm utilizing a novel design of single nanowires with GaAsSb-based multiple axial superlattices as gain medium under optical pumping. The control of lasing wavelength via compositional tuning with excellent room-temperature lasing performance is shown to result from the unique nanowire structure with efficient gain material, which delivers a low lasing threshold of ~ 6 kW/cm2 (75 μJ/cm2 per pulse), a lasing quality factor as high as 1250 and a high characteristic temperature of ~ 129 K. These results present a major advancement for the design and synthesis of nanowire laser structures, which can pave the way towards future nanoscale integrated optoelectronic systems with superior performance. © American Chemical Society 2018. This is the authors accepted and refereed manuscript to the article. Locked until 4.3.2019 due to copyright restrictions.

Electrical contacts between the nanomaterial and metal electrodes are of crucial importance both from fundamental and practical points of view. We have systematically compared the influence of contact properties by dc and EIS (Electrochemical impedance spectroscopy) techniques at various temperatures and environmental atmospheres (N2 and 1% O2). Electrical behaviors are sensitive to the variation of Schottky barriers, while the activation energy (Ea) depends on the donor states in the nanowire rather than on the Schottky contact. Equivalent circuits in terms of dc and EIS analyses could be modeled by Schottky diodes connected with a series resistance and parallel RC circuits, respectively. These results can facilitate the electrical analysis for evaluating the nanowire electronic devices with Schottky contacts.

We report on the fabrication and optoelectronic properties of p-n heterojunction arrays of p+-type Si and aligned n-type SnO2 nanowires with high rectification ratios of >104 at ±15 V. The electrical stability of the p-n heterojunction devices was improved by coating the junction with poly(methylmethacrylate) to minimize the degradation of the interface layer at the junction. As a photodiode an enhanced UV photosensitivity higher than 102 was recorded under reverse bias. Using a large forward bias in the light-emitting diode mode white light was emitted from the large-scale heterojunction devices with at least three broad peaks in the visible range, which can be attributed to the interband transitions of the injected electrons or holes mediated by an interfacial SiO2 layer with a contribution of trap-level energies. These results indicate the high potential of Si/SnO2 nanowires heterojunctions as optoelectronic devices with proper tuning of the recombination center at the junctions.

We report on the epitaxial growth of large-area position-controlled self-catalyzed GaAs nanowires (NWs) directly on Si by molecular beam epitaxy (MBE). Nanohole patterns are defined in a SiO2 mask on 2 in. Si wafers using nanoimprint lithography (NIL) for the growth of positioned GaAs NWs. To optimize the yield of vertical NWs the MBE growth parameter space is tuned, including Ga predeposition time, Ga and As fluxes, growth temperature, and annealing treatment prior to NW growth. In addition, a non-negligible radial growth is observed with increasing growth time and is found to be independent of the As species (i.e., As2 or As4) and the growth temperatures studied. Cross-sectional transmission electron microscopy analysis of the GaAs NW/Si substrate heterointerface reveals an epitaxial growth where NW base fills the oxide hole opening and eventually extends over the oxide mask. These findings have important implications for NW-based device designs with axial and radial p-n junctions. Finally, NIL positioned GaAs/AlGaAs core-shell heterostructured NWs are grown on Si to study the optical properties of the NWs. Room-temperature photoluminescence spectroscopy of ensembles of as-grown core-shell NWs reveals uniform and high optical quality, as required for the subsequent device applications. The combination of NIL and MBE thereby demonstrates the successful heterogeneous integration of highly uniform GaAs NWs on Si, important for fabricating high throughput, large-area position-controlled NW arrays for various optoelectronic device applications.

Diagnosing of the interface quality and the interactions between insulators and semiconductors is significant to achieve the high performance of nanodevices. Herein, low-frequency noise (LFN) in mechanically exfoliated multilayer molybdenum disulfide (MoS2) (~11.3 nm-thick) field-effect transistors with back-gate control was characterized with and without an Al2O3 high-k passivation layer. The carrier number fluctuation (CNF) model associated with trapping/detrapping the charge carriers at the interface nicely described the noise behavior in the strong accumulation regime both with and without the Al2O3 passivation layer. The interface trap density at the MoS2-SiO2 interface was extracted from the LFN analysis, and estimated to be Nit ~ 10(10) eV(-1) cm(-2) without and with the passivation layer. This suggested that the accumulation channel induced by the back-gate was not significantly influenced by the passivation layer. The Hooge mobility fluctuation (HMF) model implying the bulk conduction was found to describe the drain current fluctuations in the subthreshold regime, which is rarely observed in other nanodevices, attributed to those extremely thin channel sizes. In the case of the thick-MoS2 (~40 nm-thick) without the passivation, the HMF model was clearly observed all over the operation regime, ensuring the existence of the bulk conduction in multilayer MoS2. With the Al2O3 passivation layer, the change in the noise behavior was explained from the point of formation of the additional top channel in the MoS2 because of the fixed charges in the Al2O3. The interface trap density from the additional CNF model was Nit = 1.8 × 10(12) eV(-1) cm(-2) at the MoS2-Al2O3 interface.

We report on the influence of hole pattern pitch lengths in the silicon oxide mask and specific nanowire (NW) locations on the morphology and luminescence properties of self-catalyzed GaAsSb NW arrays grown by molecular beam epitaxy. Due to stronger competition for the limited amount of Ga adatoms, the GaAsSb NWs in the center of arrays with short pitch lengths possess a smaller catalyst droplet contact angle than that of the NWs at the array edge. This smaller contact angle leads to a reduction in the collection of group V flux, bringing about shorter NWs in the center. For pitch lengths beyond the diffusion length of Ga adatoms on the mask, the GaAsSb NWs are taller with larger contact angles than in the case with short pitch lengths. Considering that Sb has a longer diffusion length on the side facets of the NWs than that of As, a reduction/increase of the contact angle will bring about an increase/reduction in the Sb/As ratio of the group-V fluxes collected by the catalyst droplets. By performing micr...

The degradation pattern of SnO(2) nanowire field effect transistors (FETs) was investigated by using an individual SnO(2) nanowire that was passivated in sections by either a PMMA (polymethylmethacrylate) or an Al(2)O(3) layer. The PMMA passivated section showed the best mobility performance with a significant positive shift in the threshold voltage. The distinctive two-dimensional R(s)-μ diagram based on a serial resistor connected FET model suggested that this would be a useful tool for evaluating the efficiency for post-treatments that would improve the device performance of a single nanowire transistor.

Channel access resistance (Rsd) effects on the charge carrier mobility (μ) and low-frequency noise (LFN) in a polymethyl-methacrylate (PMMA) passivated tin-oxide nanowire (SnO2-NW) field effect-transistor were investigated. To this end, the Y function method was employed for direct electrical parameters extraction without Rsd influence. Numerical simulation was used to evaluate gate-to-channel capacitance (Cgc) accounting for the electrostatic gate coupling effects through PMMA passivation layer. Furthermore, LFN measurements were carried out to study the SnO2/dielectrics interface. The carrier number fluctuation (CNF) noise model was found appropriate to interpret LFN data provided Rsd influence is included.

The interactions between metal oxide nanowires and molecular species can significantly affect the electrical properties of metal oxide nanowires. A passivation process is needed to stabilize the electrical characteristics, regardless of the environmental changes. Herein, we investigated the passivation effects of a polymethyl methacrylate (PMMA) layer on SnO2 nanowire (NW) field-effect transistors (FETs). As a result of the PMMA coating, the electrical properties of the SnO2 NW FETs improved. The electrical noise behavior in both non-passivated and passivated devices can be described with the carrier number fluctuation model associated with the trapping and the release of charge carriers at the surface. The non-passivated devices exhibited higher noise levels than those of the passivated devices. These results demonstrate that surface passivation can lead to the suppression of dynamic responses (electron trapping/release events and scattering fluctuations).

We report on the optoelectronic properties of the aligned SnO2 nanowire (NW) field effect transistors (FETs) fabricated via a sliding transfer of NWs grown by chemical vapor deposition. Photocurrent measurements with polarized UV light confirmed a well aligned NWs along the channels. UV photosensitivity of ∼107 at the gate voltage Vg=−40 V was obtained due to a small dark-current at the turn-off state of FET. The dynamic response of the photocurrent became faster for the higher mobility SnO2 NW FETs. We expect our aligned SnO2 NW FETs will be useful as polarized UV detectors with a high sensitivity.

We have measured the local resistances in the arms and at the junction in a ZnO tetrapod separately by means of ac impedance spectroscopy. The resistance at the junction is found to be even greater than that in the arms although a volume fraction of the junction is negligibly small compared to that of the arms. This result clearly demonstrates that the junction plays a decisive role in the overall electrical property of the ZnO-tetrapod and thus determines the functionality of the ZnO tetrapod electronic devices.

In characterizing the electrical properties of individual NWs (nanowires), the amorphous oxide layer on the surface of NWs is known to limit the electrical conductivity owing to the contact barriers between metal electrodes and NWs. To remove the native oxide layer, a systematic reactive ion etching (RIE) was performed, resulting in a gradual decrease of the diameters of NWs. Voltage-current characteristics of the GaN NW devices treated by tuning the RIE process were improved as reflected by a 1000 times increase in conductance, which was in turn attributed to the removal of the thick (d∼3.5nm) contact barrier formed by the native oxide layer.