Zhisheng Wu

Needle-shaped silicon carbide nanowires: Synthesis and field electron emission properties

Bunches of needle-shaped silicon carbide (SiC) nanowires were grown from commercially available SiC powders in thermal evaporation process and using iron as catalyst. Their structure and chemical composition were studied by Raman spectroscopy and high-resolution transmission electron microscopy. The powder of these nanowires may be easily dispersed, and was used to form samples of field electron emitters. The needle shape of individual nanowires is well-suited to field electron emission. Stable emission with current density of 30.8 mA/cm2 was observed at fields as low as 9.6 V/μm, and current density of up to 83 mA/cm2 was recorded.

High hole concentration in p-type AlGaN by indium-surfactant-assisted Mg-delta doping

High hole concentration was achieved in Mg-doped AlxGa1−xN (x ∼ 0.4) by using indium-surfactant-assisted delta doping method. A maximum carrier concentration of 4.75 × 1018 cm−3 was obtained, which is three times higher than that of the conventionally delta-doped sample. Sheet resistivity as low as 2.46 × 104 Ω/sq was realized, benefiting from the high hole concentration (p). Analysis results show that the Mg incorporation is effectively enhanced, while the compensation ratio and acceptor activation energy (EA) are significantly reduced by using In surfactant. It was also found that the In surfactant may induce stronger valence-band modulation, contributing to the decrease of EA and the increase of p.

Enhancement-mode AlGaN/GaN heterostructure field effect transistors fabricated by selective area growth technique

In this letter, a method of using selective area growth (SAG) technique was proposed to fabricate the enhancement-mode (E-mode) AlGaN/GaN heterostructure field effect transistors (HFETs), which can effectively avoid the plasma treatment damage to the active region of HFETs in comparison with the conventional methods. The SAG-HFETs exhibited a good performance of the maximum drain current of 300 mA/mm and peak transconductance of 135 mS/mm with a larger positive threshold voltage of 0.4 V. The results indicate that the SAG technique is a promising method to realize the high performance E-mode GaN based HFETs.

Normally-off GaN recessed-gate MOSFET fabricated by selective area growth technique

In this letter, a normally-off GaN recessed-gate MOSFET is demonstrated using a nonplasma gate recess technique, in which the access region with AlGaN/GaN heterostructure was selectively grown on a semi-insulating GaN/Si template to naturally form a recessed gate. The normally-off recessed-gate Al2O3/GaN MOSFET presents a high threshold voltage of 3.5 V and a maximum drain current density of 550 mA/mm (at a positive gate bias of 12 V). A maximum field-effect mobility of 170 cm2 V−1 s−1 and a large on/off current ratio of more than 107 was obtained, which indicates the high quality of the Al2O3/GaN interface.

Crack-free InGaN multiple quantum wells light-emitting diodes structures transferred from Si (111) substrate onto electroplating copper submount with embedded electrodes

Crack-free InGaN multiple quantum wells (MQWs) light-emitting diodes with embedded electrode structures (EE-LEDs) were transferred from Si (111) substrate onto the electroplating copper submount. Crystalline quality was investigated by the high resolution x-ray diffraction (HR-XRD) measurement, in which no obvious deteriorations were found in the MQWs structure after the LEDs transferred from silicon substrate onto copper except for a partial residual strain relaxation in the film. The strain relaxation after silicon removal leads to a reduction in quantum confined stark effect (QCSE), which results in the enhancement of internal quantum efficiency (IQE). In comparison to the conventional LEDs on silicon substrate, the light output of the EE-LEDs on copper was enhanced by 122% at an injection current of 350 mA. Besides the enhancement of IQE, the improvement is also attributed to the following factors: the removal of the absorptive substrate, the inserting of the metal reflector between the EE-LEDs struct...

Enhanced electrostatic discharge properties of nitride-based light-emitting diodes with inserting Si-delta-doped layers

We report significantly improved electrostatic discharge (ESD) properties of InGaN/GaN-based light-emitting diodes (LEDs) with inserting Si-delta-doped layers between multiple quantum wells and n-cladding layer. The ESD endurance voltage increased from −1200 V to −4000 V with the insertion of delta-doped layers. The mechanism of the enhanced ESD properties was then investigated. According to capacitance-voltage results, the factor of capacitance modulation was ruled out. However, infrared microscopy image proved better current spreading in the LEDs with delta-doped layers. In addition, current-voltage, photoluminescence, and atomic force microscope measurements demonstrated substantial quality improvements. These two reasons were considered as the dominant mechanisms of the enhanced ESD properties.

Lateral Current Spreading Effect on the Efficiency Droop in GaN Based Light-Emitting Diodes

The lateral current spreading (CS) effect on the efficiency droop in GaN-based LEDs has been studied in terms of the CS distance (LCS) using a designed pattern with the 2-D current spreading profile. The correlations of CS effect with the electrical, luminescent and electric-thermal properties of the LEDs have been discussed. LEDs with the LCS longer than the theoretically calculated effective CS length (Leff) suffer from more serious efficiency droop and the degradation of luminescent properties. However, the influence of CS effect on the ideality factors of LEDs is not obvious. Higher chip temperature caused by poor CS was observed and may further enlarge the efficiency droop.

Emission enhancement in GaN-based light emitting diodes with shallow triangular quantum wells

GaN-based light emitting diodes (LEDs) with shallow triangular quantum wells (TQW) structure were proposed and investigated. LEDs with shallow TQW demonstrated a lower turn-on voltage and 80% higher lighting efficiency at 20 mA than devices without the shallow QW structure. A more stable emission wavelength and a lower ideality factor were achieved with the proposed structure. X-ray reciprocal space mapping revealed a partial strain relaxation in active region due to the insertion of the TQW structure. The improved performance is ascribed to the weakening of the polarization field in the MQW active region induced by the TQW structure.

Investigations of leakage current properties in semi-insulating GaN grown on Si(1 1 1) substrate with low-temperature AlN interlayers

In this work, the leakage current properties of semi-insulating GaN (SI-GaN) grown on Si(1 1 1) substrate with a low-temperature (LT) AlN interlayer were studied. Two terminal lateral current–voltage measurements revealed an early conduction phenomenon via buffer layers with an LT-AlN interlayer, in which both p- and n-type conduction phenomena were observed from Hall-effect measurement during temperature changing. It is suggested that the p-type conduction existed in the Si substrate due to the diffusion of Al atoms into the Si substrate during the epitaxial growth. The origin of n-type conduction was two-dimensional electron gas (2DEG) in the LT-AlN/GaN interface, which acted as a conduction channel in SI-GaN. Furthermore, the vertical leakage current measurement and the space charge limited current model were used to analyse the impact of the 2DEG conduction channel on the leakage current properties. It is revealed that leakage current properties are very sensitive to the thickness of the GaN layer above the LT-AlN interlayer (Top-GaN). Increasing the thickness of Top-GaN becomes an effective way to suppress leakage current. Therefore, both strain engineering and leakage current properties are essential factors to be considered in the selection of strain compensation interlayer in the growth of SI-GaN on Si substrate for power switching applications.

Enhanced Mg Doping Efficiency in P-Type GaN by Indium-Surfactant-Assisted Delta Doping Method

An indium-surfactant-assisted delta doping method is reported to enhance the hole concentration and doping efficiency of Mg-doped p-type GaN grown by metal organic chemical vapor deposition. The hole concentration is increased to 1.5×1018 cm-3 by using this method, which is 92% higher than that of conventional delta doping. This higher carrier concentration leads to an improved doping efficiency of 12%. Secondary ion mass spectroscopy reveals that the Mg incorporation is increased by the In surfactant. Photoluminescence analysis suggests that the nitrogen vacancies may be suppressed by the induced indium. Temperature-dependent Hall measurements indicate that the Mg ionization energy and compensation ratio are reduced by the In surfactant.