Nandan Singh

Wide-bandgap Zn2GeO4 nanowire networks as efficient ultraviolet photodetectors with fast response and recovery time

Ultraviolet (UV) photodetectors based on ternary Zn2GeO4 nanowire (NW) networks are demonstrated. The devices show fast response and recovery time, which is attributed to the unique NW-NW junction barrier dominated conductance for network devices. The UV-light induced barrier height modulation is much faster than the oxygen adsorption/desorption processes. The wide-band gap Zn2GeO4 NWs also exhibit high wavelength selectivity for deep UV detection. We demonstrate that ternary oxide NW-networks are ideal building blocks for nanoscale photodetectors with superior performance and facile fabrication processes.

Gold-Nanoparticle-Functionalized In2O3 Nanowires as CO Gas Sensors with a Significant Enhancement in Response

We present the room-temperature sensing of gold nanoparticle (AuNP)-functionalized In(2)O(3) nanowire field-effect transistor (NW-FET) for low-concentration CO gas. AuNPs were functionalized onto In(2)O(3) nanowires via a self-assembled monolayer of p-aminophenyltrimethoxysilane (APhS-SAM). The nanowires were mounted onto the Au electrodes with both ends in Schottky contacts. High sensor response toward low concentration of CO gas (200 ppb-5 ppm) at room temperature is achieved. The presence of AuNPs on the surface of In(2)O(3) nanowire serves to enhance the CO oxidation due to a higher oxygen ion-chemisorption on the conductive AuNP surfaces. Detailed studies showed that the sensing capabilities were greatly enhanced in comparison to those of bare nanowires or low coverage of Au NP-decorated nanowires. When the sensor is exposed to CO, the CO molecules interact with the preadsorbed oxygen ions on the AuNP surface. The CO oxidation on the AuNPs leads to the transfer of electrons into the semiconducting In(2)O(3) nanowires and this is reflected as the change in conductance of the NW-FET sensor. This work provides a promising approach for fabricating nanowire devices with excellent sensing capabilities at room temperature.

Network-enhanced photoresponse time of Ge nanowire photodetectors

We demonstrated that the photoresponse time of Ge nanowire (NW) photodetectors could be greatly improved by using percolated NW networks (instead of single NW) as the active detection channels. Although the reset time for single-NW devices was >70 s, a fast reset time <1 s was observed for NW-network devices. The enhancement was attributed to the barrier-dominated conductance for network devices, which was not available in single-NW devices. The network structures provide ideal alternative solutions to the conventional single-NW photodetectors, given their superior performances and low-cost fabrication processes.

The temperature-controlled growth of In2O3 nanowires, nanotowers and ultra-long layered nanorods.

Indium oxide (In(2)O(3)) nanowires (with diameters of 25-90 nm and lengths of 10-50 microm), nanotowers (with diameters 100-150 nm and lengths below 10 microm) and long layered nanorods (with diameters of 200-400 nm and lengths of 20-50 microm) are grown through carbothermal reduction of In(2)O(3) powder by varying the source temperature in a CVD horizontal furnace. At 875 degrees C source temperature, In(2)O(3) nanowires were formed by a VLS (vapor-liquid-solid) mechanism. The growth is changed to VS (vapor-solid)-initiated growth on the substrate surface when the source temperature is raised to 950 degrees C due to high saturation vapor pressure. Meanwhile, alternate VLS-VS grown nanotowers can be obtained at the bottom of the substrates. Growth of one-dimensional nanostructures with different structures and variation in growth mechanism from a single precursor with the confinement of vapor pressure enables the diversity of In(2)O(3) nanostructure synthesis. The morphologies and crystalline structures are characterized by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). The growth mechanisms of these structures are discussed.

Kinking-induced structural evolution of metal oxide nanowires into single-crystalline nanorings.

We report an innovative method to fabricate single-crystalline nanorings based on the conventional vapor-liquid-solid (VLS) mechanism. The controllable formation of kinks in functional oxide nanowires (NWs) can be employed to fold the VLS-grown NWs into closed ring-shaped nanostructures. Successful syntheses of single-crystalline In2O3 and Zn2GeO4 nanorings were demonstrated. The present work provides an efficient method for nanoring fabrication based on NWs. The functional metal oxide nanomaterials with unique ring-shaped structures are expected to find interesting applications such as wave-guiding and photonic circuits.

Sensing properties of different classes of gases based on the nanowire-electrode junction barrier modulation.

The role of contact between semiconducting nanowire and metal electrodes in a single nanowire field effect transistor (NW-FET) is investigated for the sensing of different type of gases. Two different types of In(2)O(3) nanowire devices, namely; Schottky contact device (SCD) and Ohmic contact device (OCD) are evaluated. SCD has shown a superior response to the reducing gas (CO) compared to oxidizing gas (NO), while OCD has shown high sensitivity towards oxidizing gas (NO) compared to the reducing gas (CO) under similar working conditions. The sensing mechanism is dominated by the contact resistance at the metal-semiconductor junction in SCD and the change in nanowire channel conductance dominates in OCD. The Schottky barrier height (SBH) was extracted using low temperature current voltage measurement which provided direct evidence for the notion that the barrier height plays a crucial role in the sensing of different types of gases. The sensing mechanism is illustrated in this work for both devices.

InAlGaAs/InP-Based Laser Photovoltaic Converter at

Interest in single-junction III-V-based semiconductor photovoltaic (PV) devices under monochromatic illumination has been renewed in recent years due to their potential to produce high conversion efficiencies. We have designed a PV converter for monochromatic power conversion at ~1070 nm, which would be ideal for Nd-/Yb-doped solid-state laser sources emitting from 1030 to 1070 nm. A single-junction PV converter is developed on InP platform using quaternary InAlGaAs Q1.1 eV as absorption material. The front grid is designed for the Gaussian nature of the incident laser beam to achieve the optimum power conversion. Preliminary tests are performed on devices under a 1070-nm laser. An open circuit voltage of 0.79 V, the current density up to 5.7 A/cm2, and the fill factor of 0.79 have been achieved. Efficiency up to 40.7% has been achieved at an incident intensity of ~0.16 W/cm2. Efficiency of these devices can be further improved by design optimization and suppressing series resistance.

\sim 1070

We report charge-compensated modified uni-traveling-carrier photodiodes (MUTC-PDs) with high photocurrent and fast response, grown using liquid group-V precursor, in an AIXTRON MOCVD system. The liquid group-V precursors involve less toxicity with better decomposition characteristics. Device fabrication is completed with standard processing techniques with BCB passivation. DC and RF measurements are carried out using a single mode fiber at 1.55 µm. For a 24-µm-diameter device (with diode ideality factor of 1.34), the dark current is 32.5 nA and the 3-dB bandwidth is ≫20 GHz at a reverse bias of 5V, which are comparable to the theoretical values. High photocurrent of over 150.0mA from larger diameter (>60 µm) devices is obtained. The maximum DC responsivity at 1.55 µm wavelength is 0.51A/W, without antireflection coating. These photodiodes play a key role in the progress of the future THz communication systems.

nm

This paper illustrates the novel approaches to augment the sensing behavior of In2O3 base nanowires towards pollutant gases using nanowire field effect transistors (NWFETs). One of the approaches is based on the method of doping. Zn-doping increases the oxygen vacancies which enhance the oxygen ion adsorption on the nanowire surface. The resultant indium zinc oxide (IZO) nanowires show a good selectivity of CO gas over NO and NO2 gas at room temperature sensing tests. Surface functionalization of the In2O3 through self-assembled monolayers can also be used to improve the sensing responses in NWFET. The metal-semiconductor interfaces with nanoscopic depletion regions create nano-Schottky barriers modulation due to the differences in work function and enhance the device conductance.