Yuefei Cai

I-V characteristics of Schottky contacts of semiconducting ZnSe nanowires and gold electrodes

The I - V behaviour of a single semiconducting ZnSe nanowire with a diameter of about 20 nm has been studied by means of electric measurement under inspection with a transmission electron microscope. The experimental results showed not only an exponential relation between the current and applied bias voltage, but also the existence of a high contact resistance with characteristic features consistent with Schottky potential barriers at the gold electrodes and the semiconducting ZnSe nanowire contacts. These Schottky barriers with asymmetrical characteristics may result from different orientations of gold grains that have different work functions. The threshold bias voltages for breakdown of the Schottky barriers examined at the Au - ZnSe nanowire contacts were about - 2.5 and 1.6 V respectively.

Metal-interconnection-free integration of InGaN/GaN light emitting diodes with AlGaN/GaN high electron mobility transistors

We report a metal-interconnection-free integration scheme for InGaN/GaN light emitting diodes (LEDs) and AlGaN/GaN high electron mobility transistors (HEMTs) by combining selective epi removal (SER) and selective epitaxial growth (SEG) techniques. SER of HEMT epi was carried out first to expose the bottom unintentionally doped GaN buffer and the sidewall GaN channel. A LED structure was regrown in the SER region with the bottom n-type GaN layer (n-electrode of the LED) connected to the HEMTs laterally, enabling monolithic integration of the HEMTs and LEDs (HEMT-LED) without metal-interconnection. In addition to saving substrate real estate, minimal interface resistance between the regrown n-type GaN and the HEMT channel is a significant improvement over metal-interconnection. Furthermore, excellent off-state leakage characteristics of the driving transistor can also be guaranteed in such an integration scheme.

Dislocation network at InN/GaN interface revealed by scanning tunneling microscopy

For heteroepitaxy of InN on GaN(0001) by molecular-beam epitaxy, the lattice misfit strain is relieved by misfit dislocations (MDs) formed at the interface between InN and GaN. Imaging by scanning tunneling microscopy (STM) of the surfaces of thin InN epifilms reveals line feature parallel to ⟨112¯0⟩. Their contrast becomes less apparent for thicker epifilms. From the interline spacing as well as a comparison with transmission electron microscopy studies, it is suggested that they correspond to the MDs beneath the surface. The STM contrast originates from both the surface distortion caused by the local strain at MDs and the electronic states of the defects.

Low-Leakage High-Breakdown Laterally Integrated HEMT-LED via n-GaN Electrode

We report lateral integration of an InGaN/GaN light-emitting diode (LED) and an AlGaN/GaN high electron mobility transistor (HEMT) via the epitaxial layers. Direct contact of the HEMT channel and the n-GaN electrode of the LED allows for conversion of a current-controlled LED to a voltage-controlled device by the gate and drain biases. The integrated HEMT-LED exhibited a light output power of 7 mW from the LED with a modulated injection current of 80 mA through the HEMT (VDD = 8 V and VGS = 1 V). The off-state breakdown voltage for the integrated HEMT-LED was 530 and 270 V at forward and reverse bias condition, respectively. The superior characteristics are attributed to the common GaN/AlN buffer platform featured with high crystalline quality and large resistivity simultaneously.

Fabrication and Characterization of High-Voltage LEDs Using Photoresist-Filled-Trench Technique

We report design, fabrication, and characterization of high-voltage LEDs (HVLEDs) using photoresist-filled-trench technique. Narrow trenches (1 μm wide) were etched to isolate the constituent LED cells of a HVLED chip and then passivated by refilling thermal curable photoresist. With a planarized HVLED surface, only a thin metal layer was needed for inter-cell connection. The narrow trench design ensured uniform light emission and allowed for negligible connection resistance. The optical power of a HVLED with 33 cells reached 1.76 W at 100 mA. The results suggested that HVLED is a viable design option for high-voltage and low current drive applications, with large and uniform light emission areas.

Monolithic integration of III-nitride voltage-controlled light emitters with dual-wavelength photodiodes by selective-area epitaxy

We report for the first time on-chip integration of III-nitride voltage-controlled light emitters with visible and ultraviolet (UV) photodiodes (PDs). InGaN/GaN and AlGaN/GaN heterostructures were grown in specific regions by selective-area epitaxy, allowing monolithic integration of versatile devices including visible light emitting diodes (LEDs), visible-light PDs, AlGaN/GaN high electron mobility transistors (HEMTs), and UV-light Schottky barrier (SB) PDs. A serial connection between the LED and HEMT through the epitaxial layers enables a three-terminal voltage-controlled light emitter (HEMT-LED), efficiently converting voltage-controlled signals into visible-light signals that can be coupled into an adjacent visible-light PD generating electrical signals. While the integrated blue HEMT-LED and PD transmits signals carried by visible light, the visible-blind SB-PD on a chip receives external UV light control signals with negligible interference from the on-chip visible-light source. This integration scheme can be extended to open an avenue for developing a variety of applications, such as smart lighting, on-chip optical interconnect, optical wireless communication, and opto-isolators.

Monolithic integration of GaN-based light-emitting diodes and metal-oxide-semiconductor field-effect transistors: comment

A potential erroneous and misleading result which is caused by the wrong calculation of current density and transconductance was found in a paper, namely [Opt. Express22, A1589 (2014)]. After checking the calculation, this paper points out the contradictory results existing in the MOSFET gate width as regards its actual layout and inconsistent I-V characteristics. Judging from the calculation result, we estimate that the author may confuse gate length with gate width in the calculation. Considering the author was conducting an interdisciplinary research and demonstrating a novel device design, these calculation errors may be trivial and comprehensible. But we still suggest that the author can make their work more accurate and comparable with other monolithic integration works by correcting the existing calculation errors.

Strain Analysis of GaN HEMTs on (111) Silicon with Two Transitional AlxGa1−xN Layers

We have designed and then grown a simple structure for high electron mobility transistors (HEMTs) on silicon, where as usual two transitional layers of AlxGa1−xN (x = 0.35, x = 0.17) have been used in order to engineer the induced strain as a result of the large lattice mismatch and large thermal expansion coefficient difference between GaN and silicon. Detailed x-ray reciprocal space mapping (RSM) measurements have been taken in order to study the strain, along with cross-section scanning electron microscope (SEM) images and x-ray diffraction (XRD) curve measurements. It has been found that it is critical to achieve a crack-free GaN HEMT epi-wafer with high crystal quality by obtaining a high quality AlN buffer, and then tuning the proper thickness and aluminium composition of the two transitional AlxGa1−xN layers. Finally, HEMTs with high performance that are fabricated on the epi-wafer have been demonstrated to confirm the success of our strain engineering and above analysis.

Low-Flicker Lighting From High-Voltage LEDs Driven by a Single Converter-Free Driver

This letter reports the design and characterization of high-power flip-chip high-voltage light-emitting diodes (HVLEDs), driven by a single converter-free LED driver for a compact and low-flicker lighting system. The HVLED chips show small forward voltage variation, uniform light emission, good optical output linearity, and stable thermal performance. All these characteristics well satisfy the demanding requirements of the novel quasi-constant power control scheme adopted in the converter-free LED driver. After bonding 12 HVLED chips (60 LED cells in total) and a single driver onto a silicon carrier through metal bonding, a low lighting flicker of 15.2%~17% was demonstrated, showing great potential for high-quality illumination applications that require low flicker, high light output, and good thermal stability.

Monolithically integrated GaN-based HEMT-LED and InGaN/GaN photodiodes for on-chip optical interconnects

Summary form only given. This paper demonstrates for the first time a monolithically integrated GaN-based voltage-controlled light emitters (HEMT-LED) and photodiodes (PD) on a GaN-on-sapphire platform for on-chip optical interconnect applications. Potentially used in an in-plane data communication system, the integrated HEMT-LED is utilized to deliver an optical signal through light emission modulated by the integrated HEMT. The integrated PD serves as a receiver of the optical signal. Excellent output characteristics without current collapse were observed for the integrated HEMT under violet light irradiation from the integrated LED. Voltage-controlled light emission from the integrated HEMT-LED was received by the on-chip PD, which exhibited modulated photocurrent current correspondingly.