InN Nanowires Research Articles

Energies of Fröhlich surface optical phonon in Q1D nanostructures: Curvature and dielectric effects

Energy of Fröhlich surface optical (SO) phonon in quasi-one-dimensional (Q1D) nanostructures remains doubtful in terms of Raman and photoluminescence experimental data. Based on a notion of the curvature proposed, the confusion is clearly clarified. It is found that the energy interval of SO modes previously accepted in the quantum system could be further divided into two sub-intervals based on the positive and negative curvature of nanowire (NW) and nanohole (NH). Furthermore, the cutoff energy and width of energy sub-intervals in NW and NH can be modulated by altering the dielectric constant of the surrounding medium. Moreover, the physical mechanism of curvature and dielectric effects on the energies of SO phonon in NW and NH are comprehended reasonably from a perspective of electrostatic potential distribution. The calculated energies of SO modes in low-energy sub-interval are fully consistent with the Raman and PL experimental results for AlN, GaN, and InN NWs. It is predicted that SO modes of high-energy sub-interval could be observed in the NH structure. The current theoretical scheme and numerical results not only extend and deepen the knowledge of the energy of the SO phonon but also can be used in the design and development of optical and optoelectronic devices based on SO modes of Q1D nanostructures.

InN nanowire solar cells on Si with amorphous Si interlayer deposited by sputtering

Here, we report the first experimental demonstration of InN nanowire solar cells deposited by RF sputtering with a bandgap energy of 1.78 eV. By adding an amorphous Si (a-Si) buffer to the n-InN/p-Si structure, we have improved the photovoltaic performance of the resulting devices while maintaining their material quality. We have firstly optimized the deposition of Si on Si(100) by DC sputtering, obtaining an amorphous material with bandgap energy of 1.39 eV. Then we have studied the influence of the thickness of the a-Si buffer layer (0–25 nm) on the structural, morphological, electrical, and optical properties of InN nanowires on Si(100) substrates. With the use of a 15-nm buffer, n-InN/a-Si/p-Si nanowire heterojunction solar cells exhibit a promising short-circuit current density of 17 mA/cm2, open circuit voltage of 0.37 V and fill factor of 35.5%, pointing to a power-conversion efficiency of 2.3% under 1-sun (AM 1.5G) illumination. These work demonstrates that the combination of in-situ sputtered a-Si, which could serve as potential passivation layer, and the light trapping enhancement by the nanostructured active layer leads to an improvement of the photovoltaic efficiency of sputtered III-nitride devices.

Self-powered flexible piezoelectric motion sensor with spatially aligned InN nanowires

The power output of piezoelectric sensors based on nanowires (NWs) is governed by various factors, but largely depends on the alignment of the NWs. However, aligning the NWs has proved to be difficult. We successfully developed self-powered and flexible piezoelectric motion sensors (PMSs) with InN NWs as a response medium. The device performance was maximized by spatially controlling the alignment of the InN NWs by applying a magnetic field, which was the first step in the development of the NW-PMSs. The output voltage of the PMS with the InN NWs aligned along the bending direction was measured to be 3.05 V, which was a significant improvement of 2.44 times compared to that with randomly-distributed NWs. A systematic analysis of the extent to which the performance of the PMSs depend on the device parameters, such as the length of the NWs, bending frequency, operation time (up to 30 days), relative humidity, and bending cycle, indicates that the device performance is sufficient for real-life applications. For example, attachment of the self-powered PMSs to human joints such as the finger, wrist, elbow, and knee revealed that these motion sensors are highly effective, thereby indicating the possibility of detecting the various motions of the human body. The self-powered PMSs developed in this work are expected to contribute to rehabilitation, disease prevention, and human–machine interaction.

Stretchable photosensor with InN nanowires operating at 1.3 µm

Stretchable photosensors, which operate in the wavelength window of 1.3 µm, were fabricated with InN nanowires (NWs) and graphene to serve as a light-absorbing medium and carrier channel, respectively. Specifically,...

Investigation of graphene/InN nanowire based mixed dimensional barristors with widely tunable Schottky barrier for highly sensitive multimodal gas sensing applications

In this work, an electrically tunable mixed dimensional heterojunction of graphene and planar InN nanowires (NW) has been investigated as a potential candidate for highly sensitive gas sensing applications. The high surface carrier concentration of InN, which prevents it from forming good rectifying contact with graphene, has been effectively suppressed through controlled partial surface oxidation in a plasma reactor. The resulting surface modified InN NWs were found to form excellent Schottky junction with graphene, with a barrier height that is widely tunable by the gate voltage, turning it into a mixed dimensional barristor device. When utilized as a multimodal molecular sensor, utilizing both capacitive and conductive modes, it exhibited very high sensitivity, with detection limits reaching down to sub-ppb range for NO2 and low ppb range for NH3 and CO, tested in both N2 and dry air environments. Additionally, surface functionalization, using hemin/benzoic acid, of this non-conventional barristor device yielded excellent selectivity for specific analytes while maintaining the high sensitivity. The responses of the sensor were found to be minimally affected with long term exposure to atmospheric humidity, which underscores the potential of this barristor device as a highly sensitive gas sensor for practical applications.

Structural and Optical Properties of InN Nanowires Formed on Si(111)

Structural and Optical Properties of InN Nanowires Formed on Si(111)

Structural, electrical and optical properties of InxGa1-xN nanowires photocathode

First-principles calculations based on density functional theory are utilized to investigate the electronic and optical properties of composition-tunable InxGa1-xN bulks and nanowires, where four different compositions (x = 0, 0.33, 0.67, 1) were considered. In the established nanowire models, the electronic properties of bare InN nanowires and passivated InN nanowires with diameters of 3.732 Å, 6.378 Å, 9.567 Å were compared. The results show that increasing the In composition could reduce the stability of the InxGa1-xN nanowires, while increasing the nanowire diameter helps to improve the stability. In addition, the band gap of InxGa1-xN nanowires narrows with the increase of In composition, and the intrinsic absorption spectrum is red-shifted. These studies will provide early design guidance for electron sources which are based on InxGa1-xN nanowire photocathodes.

Inhomogeneity of the Current Flow in High-Quality InN Nanowires

Inhomogeneity of the Current Flow in High-Quality InN Nanowires

Multi-ion scattering of charged carriers by ionized impurities in heavily doped semiconductors: From bulk to nanowires

Analytical expressions for the low-field mobility in heavily doped 3D, 2D, and 1D semiconductor structures are obtained using the quantum-kinetic approach. The study takes into account the multi-ion (M-ion) scattering of charge carriers by ionized impurities. The calculated dependences of the carrier mobility on doping concentration are compared with experiment in the heavily doped bulk materials (3D) Si, InP, GaAs, n-In0.49Ga0.51P, in heavily doped In0.15Ga0.85As quantum wells and InN nanowires, respectively. When calculating mobility in n-Si, the anisotropic effective masses of electrons in the valleys are taken into account. We explain the difference in the electron mobility of n-Si bulk crystals heavily doped by phosphorus and arsenic in the framework of the M-ion scattering model, which considers the scattering of electrons by interaction potentials with two characteristic lengths: the screening length and the effective radius of the doping ion. The number of ions M participating in the scattering process depends on the effective masses of charge carriers. For the light carriers with effective masses m<0.1m0 (m0 is the free electron mass), the two-ion (M = 2) scattering is more probable. For carriers with higher effective masses, three- and four-ion scattering is relevant.

Flexible 1.3 μm photodetector fabricated with InN nanowires and graphene on overhead projector transparency sheet.

We report the first demonstration of flexible photodetectors, operating at the wavelength window of 1.3 μm, fabricated with InN nanowires (NWs) and graphene on an overhead projector transparency (OHP) sheet. The InN NWs, used as an absorption medium for the device, were formed on a Si substrate and exhibited strong emission with a peak wavelength of 1.3 μm at room temperature. They were randomly and horizontally embedded in the graphene sandwich structure functioned as a carrier channel. The photocurrent and photoresponsivity of the flexible photodetector were found to be 1.17 mA and 0.48 A W-1, respectively, at a voltage of 1 V and a light intensity of 60 mW cm-2 of a xenon lamp. The photocurrent measured when the photodetector was bent under a strain of 3% was 1.15 mA, which corresponds to 98.3% compared to that before bending. Moreover, the photocurrent and photoresponsivity of the flexible photodetector measured after the 200 cyclic-bending tests are comparable to those measured before bending.

Self-powered image array composed of touch-free sensors fabricated with semiconductor nanowires.

We successfully develop a self-powered image array (IA) composed of 16 touch-free sensors (TFSs) fabricated with semiconductor InN nanowires (NWs) as a response medium. Without using a power supply, the InN-NW TFS can detect the position of a human hand 30 cm away from the device surface. It also distinguishes different materials such as polyimide, Al foil, printing paper, latex, and polyvinyl chloride in non-contact mode at a distance of 1 cm. The self-powered TFS-IA clearly distinguishes square-shaped transparent polydimethylsiloxane film attached to the back of a human hand positioned 5 cm from the device, indicating the possibility for detecting changes in the surface texture of human skin, such as skin burns or skin cancer. The performance of the self-powered TFS and TFS-IA is attributed to high electrostatic induction of InN NWs by external triboelectricity resulting from the simple movement of the target object, which differs markedly from conventional sensors designed to detect variations in the temperature or light essentially using a power supply.

Electrically or chemically tunable photodetector with ultra high responsivity using graphene/InN nanowire based mixed dimensional barristors

In this work, an electrically/chemically tunable highly sensitive photodetector based on mixed dimensional heterojunction of graphene and planar InN nanowires (NW) is presented. Controlled partial oxidation of InN has been employed to effectively reduce the high surface carrier concentration of InN, which normally prevents it from forming good rectifying contact with graphene. The resulting surface modified InN NWs have been found to form excellent Schottky junction with graphene, with an increase in effective Schottky barrier height (SBH) by over 1.1 eV and a ratio of forward and reverse bias currents exceeding 4 orders of magnitude. Moreover, very strong barristor (gate tunable heterojunction) action has been observed, with I on/I off ≈ 4 orders of magnitude, and SBH increase by >0.3 eV. The barristor has been demonstrated to be highly sensitive to light, especially in the ultra-voilet, visible and near IR spectra. Responsivity was found to be widely tunable by gate voltage, with the highest value exceeding 1000 A W−1. Rise and fall times being in the range of hundreds of ms are indicative of photoconductive gain, which can be attributed to the ultra high responsivity. A method of semi-permanent molecular doping has been demonstrated to realize a two-terminal version of the photodetector, where the desired responsivity can still be achieved without requiring a back gate terminal, enabling the device to be realized on insulating substrates. The effect of encapsulation has been studied as a function of time, which has showed the long term stability of the dopant-induced enhancement and ultra high responsivity of the barristor photodetector.

InN Nanowires Based Near-Infrared Broadband Optical Detector

We have reported a new method of synthesis of InN nanowires (NWs) on p-Si (100) substrate by the vapour transport cum glancing angle deposition (GLAD) technique. The field emission gun transmission electron microscopy (FEG-TEM) reveals that the InN NWs are crystalline in nature. Photoluminescence (PL) analysis shows that the optical bandgap of the NW is ~1.07 eV at 300 K. Metal-Semiconductor-Metal (M-S-M) photodetecor has been fabricated using InN NWs. The device produced trivial dark current ~7.31 μA cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> at 3 V applied voltage. The broad band near-infrared (NIR) photo-response from 900-1400 nm was recorded for the device.

Electrical Characterization of InN Nanowire/Si Heterojunctions

This report deals with the electrical properties of a heterojunction between self‐assembled InN nanowires (NWs) grown by plasma‐assisted molecular beam epitaxy and Si (111) with different doping types and concentrations. Multiple NWs are contacted in a vertical sandwich structure and current–voltage (IU) measurements show a rectifying behavior which strongly depends on the doping level of the Si substrate. Comparable results are obtained for single NWs contacted with a conductive atomic force microscope tip. The transport processes can be described by thermionic emission theory of a Schottky diode. However, field emission and recombination can not be neglected, especially for high doping concentrations of the Si substrate. The temperature dependence of the barrier height and the ideality factor have been determined by measuring IU‐curves from 200 K up to 300 K. Furthermore, electrical changes based on aging and on different environmental conditions, like contact with water, are discussed. The underlying band diagram of the InN/Si heterojunction has been simulated with the software package Nextnano++ and is in agreement with the experimental data. The simulations consider both polarities of the InN NWs and both doping types of the Si substrates.

Calculations of defect states in various sizes of InN nanowires

InN has received considerable attentions due to its small band-gap and unique properties in the III-nitride family. Understanding the effects of defects on optoelectronic properties of InN nanowire is essential for exploring its applications in future nanodevices. In this work, we have systematically calculated defect states in InN nanowires based on density-functional theory. Hydrogen passivation and several potential intrinsic point defects are considered in various sizes of nanowires, as well as charged defect states. For small-sized hexagonal nanowires, VN at N-poor condition or NIn at N-rich condition is the most stable defect. Whereas for larger-sized nanowires, VN and InN defects are competing when the N chemical potential changes, showing obvious size effect of the defect stability on the nanowire surface. Those defect states change the electronic structure of the nanowires drastically by introducing empty bands or deep level and provide possibility to tailor the optical properties in terms of forming different stable defects.

Growth of self-assembled and position-controlled InN nanowires on Si (1 1 1) by molecular beam epitaxy

This work presents a systematic growth study of self-assembled and position-controlled InN nanowires (NWs) on Si (111) substrates by plasma-assisted molecular beam epitaxy without using a buffer layer. By variation of the substrate temperature, the III/V flux ratio, and the growth time, optimal parameters for self-assembled and position-controlled NW growth are found and compared. This leads to a fundamental understanding of the underlying growth mechanism for InN NW formation. Scanning transmission electron microscopy measurements of a self-assembled InN NW show a wurtzite crystal structure with a hexagonal cross-section and flat lateral facets covered by an approximately 5 nm thick indium oxide shell. In addition, Kelvin probe force microscopy measurements indicate the existence of both lattice polarities in case of self-assembled InN NWs.

MBE Growth and Optical Properties of GaN, InN, and A3 B5 Nanowires on SiC/Si(111) Hybrid Substrate

The possibility of GaN, InN, and A3B5 nanowires MBE growth on a silicon substrate with a nanoscale SiC buffer layer has been demonstrated. Optical studies indicated a higher structural quality GaN NWs compared with the best structures of GaN NWs without silicon carbide buffer layer. The diameter of A3B5 NWs is smaller than diameter of similar NWs which were grown on a silicon substrate, because of higher lattice mismatch. In particular, InAs NWs diameter was evaluated as little as 10 nm, one of the smallest ever demonstrated for this NWs system.

Electrical characterization of Si/InN nanowire heterojunctions

We report on the electrical properties of undoped, Si-doped and Mg-doped InN nanowires measured directly on degenerate n-type and p-type Si substrates. The transport was measured with a nanoprobe technique inside a scanning electron microscope. The resulting average current density versus voltage characteristics are weakly rectifying for InN grown on n+–Si with similar ratios for all InN dopant types. On p+–Si, Mg-doped InN nanowires show a strong rectification behavior with opposite voltage polarity compared to n+–Si, while undoped and Si-doped nanowires show nearly symmetric transport. These characteristics are analyzed in terms of the properties of broken gap band offsets at the Si/InN heterojunction.

Stress-free InN nanowires grown on graphene by sublimation method

Stress-free InN nanowires (NWs) have been grown on Au/graphene/GaN/sapphire substrate by atmospheric pressure metal-In sublimation method. The phonon E2(high) and A1(LO) mode in Raman spectra of InN grown on graphene is observed at 490.8 and 585 cm−1 respectively, which is similar to that of bulk InN. The high resolution TEM (HRTEM) image of InN NWs shows that the interplanar spacing of (0002) and (11−20) are 2.85 Å and 1.77 Å, which is according with the XRD result. All the results confirm that Au-catalyst assists the growth of InN NWs, and the stress is released by the aid of the graphene inter-layer.

A Comparative Study of Vertical and Horizontal p-Type InN Nanowires Grown by MOCVD for an Optoelectronic Application

A Comparative Study of Vertical and Horizontal<i> p</i>-Type InN Nanowires Grown by MOCVD for an Optoelectronic Application