Sharif Md. Sadaf

Aluminum nitride nanowire light emitting diodes: Breaking the fundamental bottleneck of deep ultraviolet light sources

Despite broad interest in aluminum gallium nitride (AlGaN) optoelectronic devices for deep ultraviolet (DUV) applications, the performance of conventional Al(Ga)N planar devices drastically decays when approaching the AlN end, including low internal quantum efficiencies (IQEs) and high device operation voltages. Here we show that these challenges can be addressed by utilizing nitrogen (N) polar Al(Ga)N nanowires grown directly on Si substrate. By carefully tuning the synthesis conditions, a record IQE of 80% can be realized with N-polar AlN nanowires, which is nearly ten times higher compared to high quality planar AlN. The first 210 nm emitting AlN nanowire light emitting diodes (LEDs) were achieved, with a turn on voltage of about 6 V, which is significantly lower than the commonly observed 20 – 40 V. This can be ascribed to both efficient Mg doping by controlling the nanowire growth rate and N-polarity induced internal electrical field that favors hole injection. In the end, high performance N-polar AlGaN nanowire LEDs with emission wavelengths covering the UV-B/C bands were also demonstrated.

Engineering the Carrier Dynamics of InGaN Nanowire White Light-Emitting Diodes by Distributed p-AlGaN Electron Blocking Layers

We report on the demonstration of a new type of axial nanowire LED heterostructures, with the use of self-organized InGaN/AlGaN dot-in-a-wire core-shell nanowire arrays. The large bandgap AlGaN shell is spontaneously formed on the sidewall of the nanowire during the growth of AlGaN barrier of the quantum dot active region. As such, nonradiative surface recombination, that dominates the carrier dynamics of conventional axial nanowire LED structures, can be largely eliminated, leading to significantly increased carrier lifetime from ~0.3 ns to 4.5 ns. The luminescence emission is also enhanced by orders of magnitude. Moreover, the p-doped AlGaN barrier layers can function as distributed electron blocking layers (EBLs), which is found to be more effective in reducing electron overflow, compared to the conventional AlGaN EBL. The device displays strong white-light emission, with a color rendering index of ~95. An output power of >5 mW is measured for a 1 mm × 1 mm device, which is more than 500 times stronger than the conventional InGaN axial nanowire LEDs without AlGaN distributed EBLs.

Co-Occurrence of Threshold Switching and Memory Switching in

To integrate bipolar resistive switching cells into crosspoint structures, we serially connect a threshold-switching (TS) Pt/NbO₂/Pt device with a memory-switching (MS) Pt/Nb₂O₅/ Pt device and observe the suppression of the undesired sneak current. A simpler Pt/Nb₂O₅/NbO₂/Pt bilayer oxide device was designed; it simultaneously exhibits TS and MS. The unique device characteristics in the metal/oxide/metal structure can be directly integrated into a crosspoint memory array without the diode; this can significantly reduce the fabrication complexity.

\hbox{Pt}/\hbox{NbO}_{x}/\hbox{Pt}

We herein present a hybrid-type memory device in which threshold switching and bipolar resistive switching are combined. The nanoscale vanadium oxide (VOx) device simultaneously exhibited self-selective performance and memory switching by electroforming. By using W instead of Pt for the top electrode, memory performance was improved in terms of cycling, pulse endurance, and retention properties attributed to a self-formed WOx/VOx interface. Such a phenomenon in a simple metal-oxide-metal structure provides a good potential for future high-density cross-point memory devices by avoiding the sneak-path problem.

Cells for Crosspoint Memory Applications

The current LED lighting technology relies on the use of a driver to convert alternating current (AC) to low-voltage direct current (DC) power, a resistive p-GaN contact layer to inject positive charge carriers (holes) for blue light emission, and rare-earth doped phosphors to down-convert blue photons into green/red light, which have been identified as some of the major factors limiting the device efficiency, light quality, and cost. Here, we show that multiple-active region phosphor-free InGaN nanowire white LEDs connected through a polarization engineered tunnel junction can fundamentally address the afore-described challenges. Such a p-GaN contact-free LED offers the benefit of carrier regeneration, leading to enhanced light intensity and reduced efficiency droop. Moreover, through the monolithic integration of p-GaN up and p-GaN down nanowire LED structures on the same substrate, we have demonstrated, for the first time, AC operated LEDs on a Si platform, which can operate efficiently in both polarities (positive and negative) of applied voltage.

Self-Selective Characteristics of Nanoscale

In this paper, the optical and electrical properties of Mg-doped AlN nanowires are discussed. At room temperature, with the increase of Mg-doping concentration, the Mg-acceptor energy level related optical transition can be clearly measured, which is separated about 0.6 eV from the band-edge transition, consistent with the Mg activation energy in AlN. The electrical conduction measurements indicate an activation energy of 23 meV at 300 K–450 K temperature range, which is significantly smaller than the Mg-ionization energy in AlN, suggesting the p-type conduction being mostly related to hopping conduction. The free hole concentration of AlN:Mg nanowires is estimated to be on the order of 1016 cm−3, or higher.

\hbox{VO}_{x}

For the applications of resistive random access memory (RRAM), we study the complementary resistive switch (CRS) behavior of a bilayer Nb₂O_5-∞/NbO<i>y</i> RRAM. The CRS effect is explained by the redistribution of oxygen vacancies inside the two niobium oxide layers. Improved CRS effects were observed using W top electrode (TE) instead of Pt, which can be attributed to the oxygen barrier layer derived from a self-formed WO_∞ layer between the W TE and the Nb₂O_5-∞ oxide film. The niobium oxide-based CRS devices within a single memory cell can be directly integrated into a crossbar memory array without the need of extra diodes; this can significantly reduce the fabrication complexity.

Devices for High-Density ReRAM Applications

The concept of memristive filaments (MFs) is introduced, which is based on the memristors developed by the Hewlett–Packard group. The effects of key parameters on electrical properties are elucidated. The current–voltage features of bipolar and filamentary resistive switching are reproduced by using a parallel MF model with dynamic growth and rupture of multiple MFs. This model can be extended and adapted to most nanosized transition metal oxide memristors.

Alternating-Current InGaN/GaN Tunnel Junction Nanowire White-Light Emitting Diodes

Multicolor single InGaN/GaN dot-in-nanowire light emitting diodes (LEDs) were fabricated on the same substrate using selective area epitaxy. It is observed that the structural and optical properties of InGaN/GaN quantum dots depend critically on nanowire diameters. Photoluminescence emission of single InGaN/GaN dot-in-nanowire structures exhibits a consistent blueshift with increasing nanowire diameter. This is explained by the significantly enhanced indium (In) incorporation for nanowires with small diameters, due to the more dominant contribution for In incorporation from the lateral diffusion of In adatoms. Single InGaN/GaN nanowire LEDs with emission wavelengths across nearly the entire visible spectral were demonstrated on a single chip by varying the nanowire diameters. Such nanowire LEDs also exhibit superior electrical performance, with a turn-on voltage ∼2 V and negligible leakage current under reverse bias. The monolithic integration of full-color LEDs on a single chip, coupled with the capacity to tune light emission characteristics at the single nanowire level, provides an unprecedented approach to realize ultrasmall and efficient projection display, smart lighting, and on-chip spectrometer.

Optical and electrical properties of Mg-doped AlN nanowires grown by molecular beam epitaxy

We investigated the effect of a ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin film on the generation of resistive switching in a stacked Pr0.7Ca0.3MnO3 (PCMO)/Nb-doped SrTiO3 (Nb:STO) heterostructure forming a p-n junction. To promote the ferroelectric effect, the thin PZT active layer was deposited on an epitaxially grown p-type PCMO film on a lattice-matched n-type Nb:STO single crystal. It was concluded that the observed resistive switching behavior in the all-perovskite Pt/PZT/PCMO/Nb:STO heterostructure was related to the modulation of PCMO/Nb:STO p-n junction’s depletion width, which was caused either by the PZT ferroelectric polarization field effect, the electrochemical drift of oxygen ions under an electric field, or both simultaneously.