GaN-based Nanowire Research Articles

A mixer architecture using GaN-based split-gate nanowire transistor

Frequency mixer is an essential block in radio-frequency signal processing for frequency translation and phase comparison. The most common mixers are fabricated using passive elements which suffer from significant conversion loss and low isolation. Mixers using active devices are used less frequently and rather less matured on GaN technology. Here, we demonstrate a mixer based on GaN split-gate nanowire transistor, allowing low conversion loss and high isolation. A constriction is formed by electrostatic modulation of the effective gate width. The threshold voltage of the transistor is modified by one of the gate voltages through the width variation, while the other gate voltage biases the transistor in the saturation region. The nonlinear dependency of the transistor characteristics on the two gate voltages facilitates frequency translation. The mixing characteristics of this architecture are verified both experimentally and theoretically. The output power spectral density peaks at the difference frequency with a minimal conversion loss. Extremely high isolation is measured using three-port S-parameter measurements. The proposed architecture shows multiple benefits, additionally facilitating monolithic mixers on the GaN platform.

Ultralow threshold surface emitting ultraviolet lasers with semiconductor nanowires

Surface-emitting (SE) semiconductor lasers have changed our everyday life in various ways such as communication and sensing. Expanding the operation wavelength of SE semiconductor lasers to shorter ultraviolet (UV) wavelength range further broadens the applications to disinfection, medical diagnostics, phototherapy, and so on. Nonetheless, realizing SE lasers in the UV range has remained to be a challenge. Despite of the recent breakthrough in UV SE lasers with aluminum gallium nitride (AlGaN), the electrically injected AlGaN nanowire UV lasers are based on random optical cavities, whereas AlGaN UV vertical-cavity SE lasers (VCSELs) are all through optical pumping and are all with large lasing threshold power densities in the range of several hundred kW/cm2 to MW/cm2. Herein, we report ultralow threshold, SE lasing in the UV spectral range with GaN-based epitaxial nanowire photonic crystals. Lasing at 367 nm is measured, with a threshold of only around 7 kW/cm2 (~ 49 μJ/cm2), a factor of 100× reduction compared to the previously reported conventional AlGaN UV VCSELs at similar lasing wavelengths. This is also the first achievement of nanowire photonic crystal SE lasers in the UV range. Further given the excellent electrical doping that has already been established in III-nitride nanowires, this work offers a viable path for the development of the long-sought-after semiconductor UV SE lasers.

Design, Simulation and Optimization of an Enhanced Vertical GaN Nanowire Transistor on Silicon Substrate for Power Electronic Applications

A new vertical transistor structure based on GaN nanowire is designed and optimized using the TCAD-Santaurus tool with an electrothermal model. The studied structure with quasi-1D drift region is adapted to GaN nanowires synthesized with the bottom-up approach on a highly n-doped silicon substrate. The electrical performance is studied as a function of various epi-structure parameters, including region lengths and doping levels, nanowire diameter, and the impact of the surface states. The results reveal that the optimized structure has a Normally-OFF mode with a threshold voltage higher than 0.8 V and exhibits minimized leakage current, low on-state resistance, and maximized breakdown voltage. To the best of our knowledge, this is the first exhaustive study of GaN-based nanowire transistors, providing valuable insights for the scientific community and contributing to a deeper understanding of the impact of GaN nanowire parameters on device performance.

MOVPE of GaN-based mixed dimensional heterostructures on wafer-scale layered 2D hexagonal boron nitride—A key enabler of III-nitride flexible optoelectronics

We summarize our recent progress in Metal organic vapor phase epitaxy (MOVPE) van der Waals epitaxy of wafer-scale 2D layered hexagonal boron nitride (h-BN) on sapphire and subsequently grown III-N materials. This one step growth process allows for mechanical transfer of GaN-based devices from h-BN on sapphire to various supports. We first review the growth of h-BN on unpatterned and patterned sapphire templates. Second, we describe h-BN growth on dielectric pre-patterned sapphire templates, which enables dicing-free GaN-based device structures’ pick-and-place heterogenous integration of III-N devices. Third, we review the growth of self-assembled 1D GaN-based nanowire light emitting diode (LED) structures on layered 2D h-BN for mechanical transfer of nanowire LEDs. Together, these results illustrate the potential of wafer-scale van der Waals h-BN MOVPE to enhance the III-N device functionality and to improve III-N processing technology.

Influence of Growth Polarity Switching on the Optical and Electrical Properties of GaN/AlGaN Nanowire LEDs

For the development and application of GaN-based nanowire structures, it is crucial to understand their fundamental properties. In this work, we provide the nano-scale correlation of the morphological, electrical, and optical properties of GaN/AlGaN nanowire light emitting diodes (LEDs), observed using a combination of spatially and spectrally resolved cathodoluminescence spectroscopy and imaging, electron beam-induced current microscopy, the nano-probe technique, and scanning electron microscopy. To complement the results, the photo- and electro-luminescence were also studied. The interpretation of the experimental data was supported by the results of numerical simulations of the electronic band structure. We characterized two types of nanowire LEDs grown in one process, which exhibit top facets of different shapes and, as we proved, have opposite growth polarities. We show that switching the polarity of nanowires (NWs) from the N- to Ga-face has a significant impact on their optical and electrical properties. In particular, cathodoluminescence studies revealed quantum wells emissions at about 3.5 eV, which were much brighter in Ga-polar NWs than in N-polar NWs. Moreover, the electron beam-induced current mapping proved that the p–n junctions were not active in N-polar NWs. Our results clearly indicate that intentional polarity inversion between the n- and p-type parts of NWs is a potential path towards the development of efficient nanoLED NW structures.

Characteristics of GaN-Based Nanowire Gate-All-Around (GAA) Transistors.

We investigate the DC, C-V, and pulse performances in GaN-based nanowire gate-all-around (GAA) transistors with two kinds of geometry: one is AlGaN/GaN heterostructure with two dimensional electron gas (2DEG) channel and the other is only GaN layer without 2DEG channel. From I-V and C-V curves, the fabricated GaN nanowire GAA transistor with AlGaN layer clearly exhibits normally-on operation with negative threshold voltage (Vth) due to the existence of 2DEG channel on the trapezoidal shaped GaN nanowire. On the other hand, the GaN nanowire GAA transistor without AlGaN layer presents a positive Vth (normally-off operation) due to the absent of 2DEG channel on the triangle shaped GaN nanowire. However, both devices show the similar temperaturedependent I-V characteristics due to the combination of bulk channel and surface channel in GaN nanowire GAA channel are mostly contributed, rather than the 2DEG channel. GaN-based nanowire GAA transistors demonstrate to almost negligible current collapse phenomenon due to the perfect GAA gate structure in GaN nanowire. The proposed GaN-based nanowire GAA transistors are very promising candidate for both high power device and nano-electronics application.

Vertical GaN Nanowires and Nanoscale Light-Emitting-Diode Arrays for Lighting and Sensing Applications

For various lighting and monolithic sensor systems application, vertically aligned three-dimensional (3D) gallium nitride (GaN)- and indium gallium nitride (InGaN)/GaN-based LED nanowire arrays with sub-200 nm feature sizes (down to 35 nm) were fabricated using a nanosphere lift-off lithography (NSLL) technique combined with hybrid top-down etching (i.e., inductively coupled plasma dry reactive ion etching (ICP-DRIE) and wet chemical etching). Owing to the lithographic opening and well-controlled surface functionalization prior to the polystyrene nanosphere (PN) deposition, vertical GaN nanowire arrays with an area density of 9.74 × 108 cm–2 and an aspect ratio of >10 could be realized in a specified large area of 1.5 × 1.5 mm2. Optoelectrical characteristics of the nanoLEDs were further investigated in cathodoluminescence (CL) measurements, in which multiquantum well (MQW) shows a clear CL-emission at a wavelength of 465 nm. Thus, using NSLL to manufacture low-cost but highly ordered 3D GaN-based nanowir...

Low-Frequency Noise Characteristics of GaN Nanowire Gate-All-Around Transistors With/Without 2-DEG Channel

Two different lateral GaN-based nanowire gate-all-around transistors with and without 2-D electron gas (2-DEG) channel were fabricated using top-down approach, and their noise characteristics were investigated. The nanowire transistor with 2-DEG channel had a relatively larger channel cross section, which consists of regrown AlGaN/GaN plateau on the trapezoidal GaN layer, and exhibited negative threshold voltages ( ${V} _{\textsf {th}}$ ). The transistor without 2-DEG channel consisted only GaN layer with triangular-shaped smaller channel cross section and exhibited a positive ${V} _{\textsf {th}}$ . Both nanowire transistors clearly demonstrated typical $1/{f}$ noise characteristics, but the AlGaN/GaN nanowire transistor with 2-DEG channel showed larger noise magnitude. The noise characteristics of both devices are well explained by the carrier number fluctuation with correlated mobility fluctuation model. Using this model, the interface trap densities and the remote Coulomb scattering parameters were extracted, revealing a worse interface quality for the AlGaN/GaN device on the one hand, but stronger scattering for the narrow GaN transistor on the other hand.

Effect of the nanowire diameter on the linearity of the response of GaN-based heterostructured nanowire photodetectors

Nanowire photodetectors are investigated because of their compatibility with flexible electronics, or for the implementation of on-chip optical interconnects. Such devices are characterized by ultrahigh photocurrent gain, but their photoresponse scales sublinearly with the optical power. Here, we present a study of single-nanowire photodetectors displaying a linear response to ultraviolet illumination. Their structure consists of a GaN nanowire incorporating an AlN/GaN/AlN heterostructure, which generates an internal electric field. The activity of the heterostructure is confirmed by the rectifying behavior of the current–voltage characteristics in the dark, as well as by the asymmetry of the photoresponse in magnitude and linearity. Under reverse bias (negative bias on the GaN cap segment), the detectors behave linearly with the impinging optical power when the nanowire diameter is below a certain threshold (≈80 nm), which corresponds to the total depletion of the nanowire stem due to the Fermi level pinning at the sidewalls. In the case of nanowires that are only partially depleted, their nonlinearity is explained by a nonlinear variation of the diameter of their central conducting channel under illumination.

Low voltage operation of GaN vertical nanowire MOSFET

GaN gate-all-around (GAA) vertical nanowire MOSFET (VNWMOSFET) with channel length of 300 nm and diameter of 120 nm, the narrowest GaN-based vertical nanowire transistor ever achieved from the top-down approach, was fabricated by utilizing anisotropic side-wall wet etching in TMAH solution and photoresist etch-back process. The VNWMOSFET exhibited output characteristics with very low saturation drain voltage of less than 0.5 V, which is hardly observed from the wide bandgap-based devices. Simulation results indicated that the narrow diameter of the VNWMOSFET with relatively short channel length is responsible for the low voltage operation. The VNWMOSFET also demonstrated normally-off mode with threshold voltage (VTH) of 0.7 V, extremely low leakage current of ∼10−14 A, low drain-induced barrier lowering (DIBL) of 125 mV/V, and subthreshold swing (SS) of 66–122 mV/decade. The GaN GAA VNWMOSFET with narrow channel diameter investigated in this work would be promising for new low voltage logic application.

Efficient Unassisted Overall Photocatalytic Seawater Splitting on GaN-Based Nanowire Arrays

Large-scale, clean, efficient, and sustainable hydrogen production from water is one of the major goals in solar-to-fuel conversion as the sun and water represent the two most abundant and geographically balanced free resources available on earth. Considering that most of the liquid water available on the earth’s surface is present in the form of seawater, H2 generation from seawater splitting is highly desirable for large-scale practical and economical application. Herein, we report on the first demonstration of direct efficient overall solar-driven seawater splitting on p-GaN-based nanowire arrays without any external bias or sacrificial agents from various types of simulated seawater solutions. A stable solar-to-hydrogen (STH) conversion efficiency of 1.9% was obtained under concentrated irradiation, demonstrating its possible utilization for the large-scale, environmentally friendly, sustainable generation of clean solar fuel.

White light emission of monolithic InGaN/GaN grown on morphology-controlled, nanostructured GaN templates

We demonstrated an InGaN/GaN-based, monolithic, white light-emitting diode (LED) without phosphors by using morphology-controlled active layers formed on multi-facet GaN templates containing polar and semipolar surfaces. The nanostructured surface morphology was controlled by changing the growth time, and distinct multiple photoluminescence peaks were observed at 360, 460, and 560 nm; these features were caused by InGaN/GaN-based multiple quantum wells (MQWs) on the nanostructured facets. The origin of each multi-peak was related to the different indium (In) compositions in the different planes of the quantum wells grown on the nanostructured GaN. The emitting units of MQWs in the LED structures were continuously connected, which is different from other GaN-based nanorod or nanowire LEDs. Therefore, the suggested structure had a larger active area. From the electroluminescence spectrum of the fabricated LED, monolithic white light emission with CIE color coordinates of x = 0.306 and y = 0.333 was achieved via multi-facet control combined with morphology control of the metal organic chemical vapor deposition-selective area growth of InGaN/GaN MQWs.

Selective Area Sublimation: A Simple Top-down Route for GaN-Based Nanowire Fabrication.

Post-growth in situ partial SiNx masking of GaN-based epitaxial layers grown in a molecular beam epitaxy reactor is used to get GaN selective area sublimation (SAS) by high temperature annealing. Using this top-down approach, nanowires (NWs) with nanometer scale diameter are obtained from GaN and InxGa1-xN/GaN quantum well epitaxial structures. After GaN regrowth on InxGa1-xN/GaN NWs resulting from SAS, InxGa1-xN quantum disks (QDisks) with nanometer sizes in the three dimensions are formed. Low temperature microphotoluminescence experiments demonstrate QDisk multilines photon emission around 3 eV with individual line widths of 1-2 meV.

(Invited) Phosphor-Free III-Nitride Nanowire White Light Emitting Diodes: Challenges and Prospects

Tremendous progress has been made in GaN-based nanowire light-emitting diodes (LEDs), which can exhibit drastically reduced dislocation densities and polarization fields, compared to the conventional quantum well devices. However, the performance of nanowire LEDs is limited due to the poor hole transport, the electron overflow, and the large surface nonradiative recombination. In this context, we have investigated the molecular beam epitaxial growth, fabrication and characterization of full-color InGaN/(Al)GaN dot-in-a-wire LEDs on Si substrates, wherein the emission characteristics are controlled by the dot properties in a single epitaxial step. With the use of p-type modulation doping, electron blocking layer (EBL), and core-shell heterostructures, we have further demonstrated superior performance phosphor-free white LEDs that can exhibit record high output power of ~ 5.2 mW and unprecedentedly high color rendering index (CRI) of > 95.

Highly ordered GaN-based nanowire arrays grown on patterned (100) silicon and their optical properties

Arrays of GaN-based nanowires have been synthesized on patterned silicon without a catalyst. The spatial density, length and average radius of the nanowires can be well-controlled. The GaN core contains two semipolar facets and a controllable polar facet. The nanowire heterostructures exhibit excellent laser behavior.

Single GaN-Based Nanowires for Photodetection and Sensing Applications

We present a study of the structural properties of GaN/AlN heterostructured nanowires grown by plasma-assisted molecular-beam epitaxy. We combine transmission electron microscopy measurements with theoretical calculations of the strain distribution and band diagram in order to gain understanding of the potential distribution in these nanostructures. The effects of surface states and of the formation of a core–shell heterostructure are discussed. The structural properties are correlated with the performance of GaN-based nanowire photodetectors and sensors. In particular, we discuss a sensor design where the insertion of an AlN barrier in a GaN nanowire is proposed as a material engineering solution to inhibit the electron transport through the core, confining the current close to the nanowire sidewalls.

Investigation of the wet-etching mechanism of Ga-polar AlGaN/GaN micro-pillars

The wet-etching mechanism of Ga-polar AlGaN/GaN micro-pillars is analyzed in this work. By using solutions of KOH in ethylene glycol (KE), we discover the difference in wet-etching processes with and without an AlGaN layer. In the Ga-polar micro-pillar GaN-based nano-wire, a micro-pillar with {1−100} facet and a pyramid with a {11−212} facet are realized by accurately controlling the wet-etching rate and time, respectively. Schematic drawings are established to explain the dissimilarity of these two structures. Our research attempts to fabricate lager scale nano-wires and quasi-polarity substrates.

Polaron effect on linear and nonlinear intersubband optical absorption of a wurtzite GaN-based nanowire

Based on the density matrix approach and the perturbation treatment, the polaronic effect on the linear and nonlinear intersubband optical absorption coefficients in quasi-one-dimensional wurtzite nanowires (NWs) is investigated. The simple analytical formulas for polaronic states and the linear and nonlinear optical absorption coefficients in the NW systems are also deduced. Numerical calculation on a freestanding wurtzite GaN NW is performed. The polaronic effect and quantum size effect as well as the influence of incident optical intensity on the optical absorption properties are analyzed and discussed. The results show that, the polaronic effect results in significant increase of the absorption coefficients, and obvious decrease of saturation absorption intensity. In order to observe strong optical absorption, a nitride NW with relative small radius should be chosen. Moreover, a comparison with the situation of GaN-based quantum well systems is also done, and the profound physics is analyzed reasonably. • Polaronic states including the IO and QC phonon modes in wurtzite nitride NWs are obtained. • Polaronic effect results in significant increase of the linear and nonlinear absorption coefficients. • Taking the polaronic effect into account, saturation absorption intensity decreases obviously. • GaN-based NWs with smaller radius should be chosen for observing stronger optical absorption.

Nonradiative Recombination Mechanism in Phosphor-Free GaN-Based Nanowire White Light Emitting Diodes and the effect of Ammonium Sulfide Surface Passivation

We report on a detailed investigation of nonradiative recombination processes in InGaN/GaN dot-in-a-wire white light emitting diodes (LEDs) grown by molecular beam epitaxy. It is observed that, for such nanowire LEDs, the peak quantum efficiency occurs at relatively high injection current levels of >100 A/cm2, compared to those of conventional InGaN/GaN quantum well blue LEDs. Through detailed simulation studies, it is concluded that, Shockley-Read-Hall nonradiative recombination, due to the presence of surface states and defects, plays a dominant role on the nanowire LED performance. We have further shown that such nonradiative recombination can be greatly reduced when the nanowire LEDs are treated by ammonium sulfide solution ((NH4)2Sx, 40-48% concentration), which can lead to a much faster increasing trend of quantum efficiency vs. injection current, compared to the unpassivated devices.

Nonradiative Recombination Mechanism in Phosphor-Free GaN-Based Nanowire White Light Emitting Diodes and the effect of Ammonium Sulfide Surface Passivation

not Available.