Shyh-Jer Huang

Efficiency enhancement in ultraviolet light-emitting diodes by manipulating polarization effect in electron blocking layer

The characteristics of the ultraviolet light-emitting diode (LED) with conventional and specifically designed electron blocking layers (EBLs) are investigated numerically and experimentally in this work. Simulation results show that delicately designed EBLs can not only capably perform the electron blocking function but also eliminate the incidental drawback of obstruction of hole injection caused by the nature of the large polarization field at the c-plane nitride heterojunction. It is shown that the polarization induced downward band bending can be mitigated when the portion of conventional EBL lying adjacent to the active region is replaced by a graduated AlGaN layer. The conduction band profile indicates that this replacement structure could have the capability of electron confinement similar to the conventional structure, and the valence band profile indicates that the spike induced by the polarization field is simultaneously eliminated, assisting the process of hole injection and distribution in the...

Enhanced Performance of Nitride-Based Blue LED With Step-Stage MQW Structure

A step-stage InGaN/GaN multiquantum-well (MQW) structure can enhance the efficiency of GaN-based light-emitting diodes (LEDs). Compared to dual-stage MQW LEDs, the step-stage MQW LEDs have lower forward voltage and higher light output. The measured light output power of step-stage LEDs operating at 350 mA shows an increase of approximately 23% with an external quantum efficiency (EQE) increase of 6.6%, when compared to dual-stage LEDs.

Improvement of Light Intensity for Nitride-Based Multi-Quantum Well Light Emitting Diodes by Stepwise-Stage Electron Emitting Layer

We demonstrate the improvement of light intensity for light emitting diodes (LEDs) using an electron emitting layer (EL) constructed by quantum wells (QWs) with stepwise depths. The results show that the stepwise-stage EL can further improve the light intensity, about 20% increment compared with that of a dual-stage LED at 20 mA. It is found that the crystal quality and electron capture rate are higher for a stepwise-stage structure. We also explore the influence of the number of QWs in the stepwise-stage EL on the light intensity. The result shows that the intensity increases with QW number but the saturation starts at about eight QWs.

Tailoring of polarization in electron blocking layer for electron confinement and hole injection in ultraviolet light-emitting diodes

The influence of the AlGaN electron blocking layer (EBL) with graded aluminum composition on electron confinement and hole injection in AlGaN-based ultraviolet light-emitting diodes (LEDs) are investigated. The light output power of LED with graded AlGaN EBL was markedly improved, comparing to LED with conventional EBL. In experimental results, a high increment of 86.7% can be obtained in light output power. Simulation analysis shows that via proper modification of the barrier profile from the last barrier of the active region to EBL, not only the elimination of electron overflow to p-type layer can be achieved but also the hole injection into the active region can be enhanced, compared to a conventional LED structure. The dominant factor to the performance improvement is shown to be the modulation of polarization field by the graded Al composition in EBL.

Direct Growth of a-Plane GaN on r-Plane Sapphire by Metal Organic Chemical Vapor Deposition

In this study, we had demonstrated the direct growth of nonpolar a-plane GaN on an r-plane sapphire by metal organic chemical vapor deposition (MOCVD) without any buffer layer. First, in this experiment, we had determined the optimum temperature for two-step growth, including obtaining three-dimensional (3D) GaN islands in the nucleation layer and coalescing with a further two-dimensional (2D) growth mode. The result shows that the nucleation layer grown under high temperature (1150 °C) leads to large islands with few grain boundaries. Under the same temperature, the effect of the V/III ratio on the growth of the overlaying GaN layer to obtain a flat and void free a-plane GaN layer is also studied. The result indicates one can directly grow a smooth epitaxial layer on an r-plane sapphire by changing the V/III ratio. The rms roughness decreases from 13.61 to 2.02 nm. The GaN crystal quality is verified using a mixed acid to etch the film surface. The etch pit density (EPD) is 3.16 ×107 cm-2.

Scalability of Phase Change Materials in Nanostructure Template

The scalability of In2Se3, one of the phase change materials, is investigated. By depositing the material onto a nanopatterned substrate, individual In2Se3 nanoclusters are confined in the nanosize pits with well-defined shape and dimension permitting the systematic study of the ultimate scaling limit of its use as a phase change memory element. In2Se3 of progressively smaller volume is heated inside a transmission electron microscope operating in diffraction mode. The volume at which the amorphous-crystalline transition can no longer be observed is taken as the ultimate scaling limit, which is approximately 5 nm3 for In2Se3. The physics for the existence of scaling limit is discussed. Using phase change memory elements in memory hierarchy is believed to reduce its energy consumption because they consume zero leakage power in memory cells. Therefore, the phase change memory applications are of great importance in terms of energy saving.

Dislocation reduction through nucleation and growth selectivity of metal-organic chemical vapor deposition GaN

A novel serpentine channel structure is used to mask the sapphire substrate for the epitaxial growth of dislocation-free GaN. Compared to the existing epitaxial lateral overgrowth methods, the main advantages of this novel technique are: (a) one-step epitaxial growth; (b) up to 4 times wider defect-free regions; and (c) the as-grown GaN film can be transferred easily to any type of substrate. TEM, etch pits and cathodoluminescence experiments are conducted to characterize the quality of as-grown GaN. The results show that the average etch-pit density in the yet-to-be-optimized GaN epi-layers is about 4 × 105 cm−2. The underlying physics of selective nucleation and growth is investigated using the finite element method (COMSOL). It is concluded that the proximity effect dominates the selective growth of GaN on the serpentine channel structure masked sapphire. This novel technique is a promising candidate for the growth of high quality III-nitride and the subsequent high-performance device fabrication inclu...

Normally-off AlGaN/GaN high-electron-mobility transistor on Si(111) by recessed gate and fluorine plasma treatment

We have studied the efficiency of using both recessed gate and fluorine plasma treatment to achieve normally-off high-electron-mobility transistor (HEMT). It is found that, by a simple recess process, one cannot achieve normally off device with high drain current because of gate leakage problem after inductively coupled plasma (ICP) etching for recessed structure. The proper method is adding fluorine treatment based on recess gate. The normally off GaN HEMTs with recess gate and fluorine treatment show very good performance. It is found that the threshold voltages can be shifted to +1.1 V, and the drain current at VGS − Vth = 2 V and VDS = 20 V was 218 mA/mm.

Suppression of Nonradiation Recombination by Selected Si Doping in AlGaN Barriers for Ultraviolet Light-Emitting Diodes

The effect of selective Si doping on the emission efficiency in ultraviolet (UV) light-emitting diodes (LEDs) is investigated both experimentally and theoretically. The results show that the light output power increases with the number of Si-doped barriers (QBs). Experimental results indicate that compared with an all-undoped-QB LED, a factor of 3.17 can be achieved for the output power of an all-doped-QB LED at 350 mA. Detailed analysis on this phenomenon shows that the Si-doped QB is beneficial to suppress the nonradiative recombination rate by excess electrons in doped barriers.

Atomic-Monolayer Two-Dimensional Lateral Quasi-Heterojunction Bipolar Transistors with Resonant Tunneling Phenomenon

High-frequency operation with ultrathin, lightweight, and extremely flexible semiconducting electronics is highly desirable for the development of mobile devices, wearable electronic systems, and defense technologies. In this work, the experimental observation of quasi-heterojunction bipolar transistors utilizing a monolayer of the lateral WSe2-MoS2 junctions as the conducting p-n channel is demonstrated. Both lateral n-p-n and p-n-p heterojunction bipolar transistors are fabricated to exhibit the output characteristics and current gain. A maximum common-emitter current gain of around 3 is obtained in our prototype two-dimensional quasi-heterojunction bipolar transistors. Interestingly, we also observe the negative differential resistance in the electrical characteristics. A potential mechanism is that the negative differential resistance is induced by resonant tunneling phenomenon due to the formation of quantum well under applying high bias voltages. Our results open the door to two-dimensional materials for high-frequency, high-speed, high-density, and flexible electronics.