GaAs-based Nanowire Research Articles

Investigation of GaAs-based nanowire heterostructures using tomography based on STEM-HAADF tilt-series [1]

Investigation of GaAs-based nanowire heterostructures using tomography based on STEM-HAADF tilt-series [1]

Current timer switch in a GaAs-based nanowire coupled with polyoxometalate nanoparticle and conductive AFM tip

We demonstrate a current timer switch function in a GaAs-based nanowire electrostatically coupled with a polyoxometalate nanoparticle (POM NP) and a conductive atomic force microscopy (AFM) tip. The nanowire current associated with the charge state of the POM NP on the nanowire surface abruptly changed after several ten seconds from biasing the conductive tip. The current switch timing changed depending on the tip voltage. The timer switch function appeared when the tip approached the POM NP where the AFM phase image showed phase lag at high atmospheric humidity condition. We discuss the timer mechanism in terms of the configuration of the electromechanical potential of the cantilever and dynamic potential modulation by mobile ions and polarized molecules in the water-absorbed POM NP surface.

Optical property and lasing of GaAs-based nanowires

GaAs-based nanowire (NW) lasers working in the infrared region is critical in integrated optoelectronics. In the past few decades, the field of NW lasers has developed rapidly. Compared with materials working in the ultraviolet and visible ranges, GaAs-based infrared NW lasers, however, are more difficult to achieve because of their specific properties. In this review, we focus on the recent developments of GaAs-based NWs, more especially, the optical property and lasing of GaAs-based NWs. The growth mechanism of GaAs NWs is introduced in detail, including the crystal phase control and the growth of complex structures. Subsequently, the influence and improvement of the optical properties of GaAs-based NWs are introduced and discussed. Finally, the design and latest progress of GaAs-based NW lasers are put forward.

Enhanced Photoresponsivity of a GaAs Nanowire Metal-Semiconductor-Metal Photodetector by Adjusting the Fermi Level.

Metal-semiconductor-metal (MSM)-structured GaAs-based nanowire photodetectors have been widely reported because they are promising as an alternative for high-performance devices. Owing to the Schottky built-in electric fields in the MSM structure photodetectors, enhancements in photoresponsivity can be realized. Thus, strengthening the built-in electric field is an efficacious way to make the detection capability better. In this study, we fabricate a single GaAs nanowire MSM photodetector with superior performance by doping-adjusting the Fermi level to strengthen the built-in electric field. An outstanding responsivity of 1175 A/W is obtained. This is two orders of magnitude better than the responsivity of the undoped sample. Scanning photocurrent mappings and simulations are performed to confirm that the enhancement in responsivity is because of the increase in the hole Schottky built-in electric field, which can separate and collect the photogenerated carriers more effectively. The eloquent evidence clearly proves that doping-adjusting the Fermi level has great potential applications in high-performance GaAs nanowire photodetectors and other functional photodetectors.

Formation and characterization of charge coupled structure of polyoxometalate particles and a GaAs-based nanowire for readout of molecular charge states

To investigate the molecular charge dynamics of polyoxometalate (POM) molecules, we formed and characterized a charge coupled structure with POM molecular particles and a GaAs-based nanowire. In our system, the charge sensitivity was locally increased by capacitive coupling between a metal tip and the POM particle. Surface dispersion of POM particles on the GaAs nanowire was carried out in a controlled manner by choosing an appropriate solvent and POM concentration. We found that, after POM surface dispersion, the current in the GaAs nanowire remarkably increased by charging the POM particles using a conductive atomic force microscopy tip. The current change strongly depended on humidity of the measurement environment. The nanowire current under capacitive coupling between the conductive tip and the POM particle on the nanowire surface showed steps with a height of approximately 70 nA, suggesting that multiple hole charging and discharging occurred in the particle in a synchronized manner.

Analytical derivation of charge relaxation time distribution in transistor from current noise spectrum using inverse integral transformation method

An analytical technique is proposed to reveal the relaxation time distribution of dynamic charge events using the current noise spectrum of a transistor, by applying an inverse integral transformation to the McWhorter model. In the proposed method, the continuous relaxation-time distribution function G(τ) can be analytically derived from the noise spectra S(ω) without a spectrum deconvolution. The feasibility of the proposed method is demonstrated by characterizing the charge dynamics of tetraphenylporphyrin molecules dispersed on the surface of a GaAs-based nanowire field-effect transistor. Our analysis successfully verified the time constant of the molecule-related dynamic charge events and effects of photo-excitation.

Direct Coupling of Coherent Emission from Site-Selectively Grown III–V Nanowire Lasers into Proximal Silicon Waveguides

Semiconductor nanowire (NW) lasers are nanoscale coherent light sources that exhibit a small footprint, low-threshold lasing characteristics, and properties suitable for monolithic and site-selective integration onto Si photonic circuits. An important milestone on the way toward novel on-chip photonic functionalities, such as injection locking of laser emission and all-optical switching mediated by coherent optical coupling and feedback, is the integration of individual, deterministically addressable NW lasers on Si waveguides with efficient coupling and mode propagation in the underlying photonic circuit. Here, we demonstrate the monolithic integration of single GaAs-based NW lasers directly onto lithographically defined Si ridge waveguides (WG) with low threshold power densities of 19.8 μJ/cm2 when optically excited. The lasing mode of individual NW lasers is shown to couple efficiently into propagating modes of the underlying orthogonal Si WG, preserving the lasing characteristics during mode propagati...

Detection of charge dynamics of a tetraphenylporphyrin particle using GaAs-based nanowire enhanced by particle–metal tip capacitive coupling

We investigate a detection technique of charge dynamics of a molecular particle using a GaAs-based nanowire where the charge sensitivity is locally enhanced by particle–metal tip capacitive coupling. By equivalent circuit analysis, it was clarified that the nanowire channel potential becomes sensitive to the molecular particle on the nanowire when the particle is capacitively coupled with a metal tip. The concept was demonstrated using a GaAs-based nanowire with tetraphenylporphyrin (TPP) particles on its surface and a measurement system integrating an atomic force microscope (AFM) and a dynamic current measurement monitor/spectrum analyzer. When the metal tip was in contact with a TPP particle on the nanowire under an appropriate tip bias condition, random telegraph signal (RTS) noise was imposed on the nanowire current, suggesting the increase in sensitivity to the charge state of the particle by the metal tip contact. We discussed the origin of the RTS noise through analysis of the time constant of RTS noise, RTS amplitude, and noise spectrum.

Implementation of a noise-coexistence threshold logic architecture on a GaAs-based nanowire FET network

Toward reconfigurable and noise-coexistence information processing system utilizing nanostructures, we study a threshold logic circuit and a double threshold function using a GaAs-based nanowire field-effect transistor (FET) network. A noise coexistence capability is based on a noise-assisted state transition in a threshold function in a threshold logic element. We fabricate a circuit reconfigurable between NAND and NOR functions. A hysteresis transfer characteristic with double threshold is realized in the GaAs nanowire by using a silicon nitride (SiN) as the gate insulator. We introduce a unique inverter design using the SiN-gate FET as a load to achieve the transfer characteristic with clockwise hysteresis, similar to a Schmitt trigger.

Detection of discrete surface charge dynamics in GaAs-based nanowire through metal-tip-induced current fluctuation

We investigated the detection of discrete charge dynamics of an electron trap in a GaAs-based nanowire surface through current fluctuation induced by a metallic scanning probe tip. An equivalent circuit model indicated that the charge state in the surface strongly reflects the channel potential when the local surface potential is fixed by the metal tip, which suggests that random charging and discharging dynamics of the trap appears as random telegraph signal (RTS) noise in the nanowire current. Experimental demonstration of the concept was carried out using a GaAs-based nanowire and an atomic force microscope (AFM) system with a conductive tip. We observed the RTS noise in the drain current and superposition of the Lorentzian component in the noise spectrum when the metal tip was in contact with the nanowire surface at specific positions. The obtained results indicate the possibility of detecting charge dynamics of the individual surface trap in semiconductor devices.

Detection of molecular charge dynamics through current noise in a GaAs-based nanowire FET

The detection of static and dynamic molecular charge states using a GaAs-based nanowire field-effect transistor (FET) was investigated. Tetraphenylporphyrin (TPP) was put on the device as target molecules. After coating TPP on the FET, the drain current clearly decreased. On the other hand, the current largely increased by 405-nm light irradiation, indicating that TPP worked as a photo-excited donor. The light irradiation on the FET also induced a Lorentzian noise component, which was superimposed onto conventional 1/f noise. These behaviors were not seen in the gateless nanowire even with TPP. The obtained results indicated that electrical interaction between TPP and the nanowire was enhanced when a metal gate existed, although the channel was protected from TPP by the gate metal. We discuss the observed behaviors on the basis of a model where only TPP in the gate periphery modulated the channel potential and the drain current.

InGaAs axial-junction nanowire-array solar cells

Axial p–i–n junction nanowire (NW) solar cells (SCs) with a position-controlled GaAs-based NW array were fabricated by selective-area metal organic vapor phase epitaxy (SA-MOVPE). The measured electron-beam-induced current (EBIC) signals showed the formation of an axial p–i–n junction, which confirms power generation under sunlight illumination. The series resistance of the NW SCs is much higher than that of conventional planar SCs based on Si or other III–V compound semiconductors. The main difficulty concerning the fabrication of these NW SCs is the degradation of series resistance between the GaAs-based NWs and the indium–tin oxide (ITO) deposited as a transparent electrode. The series resistance of the fabricated GaAs-based NW SCs was reduced by introducing a tin doping contact layer between the ITO and the NW array, which is formed by pulse doping. As a result of this improved structure, the fabricated SCs exhibited an open-circuit voltage of 0.544 V, a short-circuit current of 18.2 mA/cm2, and a fill factor of 0.721 for an overall conversion efficiency of 7.14% under AM1.5G illumination. The series resistance of the SCs could be decreased to 0.132 Ω·cm2, which is one order of magnitude lower than that of the SC without a highly doped contact layer. This reduced series resistance indicates that nanostructure SCs with transparent electrodes and multijunction NW SCs with high efficiencies can be fabricated on a commercial basis in the near future.

Detection of weak biological signal utilizing stochastic resonance in a GaAs-based nanowire FET and its parallel summing network

We investigated biological signal detection utilizing stochastic resonance (SR) in a GaAs-based nanowire field-effect transistor (FET) and its parallel summing network. The SR phenomenon, in which the response to a weak signal is optimized by noise, was caused in the nanowire FET having a nonlinear transfer characteristic. We confirmed that the SR occurred on weak aperiodic electromyogram (EMG) signals induced on the surface of the forearm of a human subject. The EMG contaminated with intrinsic noise from the body was successfully detected in a parallel summing network of nanowire FETs. The output signal-to-noise ratio (SNR) of the nanowire FET network was higher than those of the conventional detection techniques. We also showed that the FET network with multiple threshold voltages achieved a high SNR in a wide range of EMG intensities.

Performance-limiting factors for GaAs-based single nanowire photovoltaics.

GaAs nanowires (NWs) offer the possibility of decoupling light absorption from charge transport for high-performance photovoltaic (PV) devices. However, it is still an open question as to whether these devices can exceed the Shockley-Queisser efficiency limit for single-junction PV. In this work, single standing GaAs-based nanowire solar cells in both radial and vertical junction configurations is analyzed and compared to a planar thin-film design. By using a self-consistent, electrical-optically coupled 3D simulator, we show the design principles for nanowire and planar solar cells are significantly different; nanowire solar cells are vulnerable to surface and contact recombination, while planar solar cells suffer significant losses due to imperfect backside mirror reflection. Overall, the ultimate efficiency of the GaAs nanowire solar cell with radial and vertical junction is not expected to exceed that of the thin-film design, with both staying below the Shockley-Queisser limit.

Characterization of GaAs-Based Three-Branch Nanowire Junction Devices by Light-Induced Local Conductance Modulation Method

Nonlinear voltage transfer characteristics in GaAs-based three-branch nanowire junction (TBJ) devices were investigated by a light-induced local conductance modulation method. In this measurement system, the conductance in the device was locally increased by focused laser light irradiation. The nonlinear transfer curve was greatly changed when the laser light was irradiated on the positively biased branch. The conductance domain was found to exist at the end of the positively biased branch of the TBJ by scanning the light position. When a SiNx thin layer was deposited on the nanowire surface, the surface potential was increased and the nonlinearity in the transfer curve was reinforced simultaneously. The obtained results suggest that the asymmetric channel depletion model is appropriate for the observed nonlinearity mechanism in the GaAs TBJ at room temperature.

GaAs-Based Nanowire Devices with Multiple Asymmetric Gates for Electrical Brownian Ratchets

GaAs-based nanowire devices having multiple asymmetric gates for electrical Brownian ratchets were fabricated and characterized. From three-dimensional potential simulation results and current–voltage characteristics, we confirmed the formation of the asymmetric potential in our device design. Direct current was generated at room temperature by repeatedly switching the potential in a multiple-asymmetric-gate device on and off. Such current was not observed in either a single-asymmetric-gate device or a multiple-symmetric-gate device. The current direction and input frequency dependences of the net current indicated that the observed current was generated by the flashing-ratchet mechanism.

Novel Nanowire-Based Flip-Flop Circuit Utilizing Gate-Controlled GaAs Three-Branch Nanowire Junctions

A novel set-reset flip-flop (SR-FF) circuit integrating gate-controlled GaAs three-branch nanowire junctions (TBJs) is designed, fabricated, and characterized. Fundamental logic gates including AND, NOT, and NAND are constructed using Schottky wrap gate (WPG)-controlled TBJs together with inverter circuits that have the same configuration. The present SR-FF circuit is simply designed using a pair of cross-coupled TBJ-based NAND gates. The circuit is successfully fabricated on a GaAs-based hexagonal nanowire network. Its correct operation with a voltage transfer gain larger than unity is demonstrated. Reduction of circuit area and possible operation speed are also discussed.

Characterization of Low-Frequency Noise in GaAs Nanowire Field-Effect Transistors Controlled by Schottky Wrap Gate

Low-frequency noise in GaAs-based nanowire field-effect transistors (FETs) controlled by a Schottky wrap gate (WPG) is investigated focusing on the size dependence of 1/f noise and the basic behavior of a gentle slope of the noise spectrum at a relatively high frequency. 1/f noise is found to systematically depend on the nanowire width W and gate length LG, which is explained by the conventional flicker noise model. The evaluated flicker noise coefficient KF is on the order of 10-23 V2 F, comparable to that of Si metal–oxide–semiconductor (MOS) FETs. The gentle slope close to 1/f0.5 frequently appears in the noise spectrum from the fabricated devices. Its intensity is found to be proportional to gate leakage current, suggesting that electrons flowing through the AlGaAs barrier layer induce generation-recombination (GR) noise in the gate region.

Programmable nano-switch array using SiN/GaAs interface traps on a GaAs nanowire network for reconfigurable BDD logic circuits

Programmable nano-switch arrays on GaAs-based nanowire networks are investigated for a reconfigurable binary-decision-diagram (BDD) logic circuit. A programmable switch was simply realized by inserting a SiNx thin layer between a metal gate and a nanowire. Fabricated switches were characterized in terms of hysteresis curve, program time dependences of off-state retention time, and on-state current. HCl treatment on SiNx prior to metal gate formation was found to remarkably improve the switching characteristics. We experimentally demonstrated correct and stable operation of a four-input reconfigurable BDD circuit integrating the switch array with HCl treatment.

Voltage Transfer Characteristics in GaAs-Based Three-Branch Nanowire Junctions Controlled by Schottky Wrap Gates

Nonlinear voltage transfer characteristics in GaAs-based three-branch nanowire junctions (TBJs) controlled by Schottky wrap gates (WPGs) are investigated by characterization of the gate voltage and size dependences in detail. WPGs squeezed the nanowires only in the node portion and modulated the curve only in the low-input-voltage region. When the entire nanowire was narrowed geometrically, the voltage transfer curve became abrupt in a wide voltage range. On the other hand, the nanowire length affected only the curve in the high-input-voltage region. These results indicate that the voltage transfer characteristics of the WPG-controlled TBJ device in the low- and high-voltage regions are controlled by the junction node with WPGs and the end of the positively biased nanowire, respectively. The observed behaviors can be understood in terms of a surface-potential-induced field domain model.