Peng Ma

Theoretical study of polarization-doped GaN-based light-emitting diodes

Insufficient hole injection is a major impediment to the luminescence efficiency of III-nitride light-emitting diodes (LEDs). In our previous work by Zhang et al. [Appl. Phys. Lett. 97, 062103 (2010)], high-density mobile three-dimensional hole gas is obtained in Mg-doped Al composition graded AlGaN layer with Al composition linearly decreasing from a certain value to 0. In this paper, it is revealed by a theoretical study that the hole injection efficiency in blue-light GaN-based LEDs can be greatly enhanced by using this polarization-doped method. An increase in the electroluminescence intensity and the internal quantum efficiency in polarization-doped GaN-based LEDs is observed, in comparison with a conventional LED.

The anistropy of field effect mobility of CVD graphene grown on copper foil.

Through adjusting the focal plane of the optical microscope, we have verifi ed that these trenches on the copper foil have a con-cave cross section. It is similar with the surface topography of copper foil, whose optical image is shown in the inset panel of Figure 1 a. The surface of copper foil shows a directional texture consisting of many parallel trenches with spacing on the order of micrometers, these trenches are considered to be produced during the foil rolling process used to fabricate the Cu foil, with the trenches running parallel to the shear/drawing direction.

Spatial variation of optical and structural properties of ELO GaN directly grown on patterned sapphire by HVPE

A 275 mu m thick GaN layer was directly grown on the SiO2-prepatterned sapphire in a home-built vertical hydride vapour phase epitaxy (HVPE) reactor. The variation of optical and structure characteristics were microscopically identified using spatially resolved cathodeluminescence and micro-Raman spectroscopy in a cross section of the thick film. The D X-0(A) line with the FWHM of 5.1 meV and etch- pit density of 9 x 10(6) cm(-2) illustrated high crystalline quality of the thick GaN epitaxial layer. Optically active regions appeared above the SiO2 masks and disappeared abruptly due to the tapered inversion domains at 210 - 230 mu m thickness. The crystalline quality was improved by increasing the thickness of the GaN/sapphire interface, but the region with a distance of 2 mu m from the top surface revealed relatively low quality due to degenerate surface reconstruction by residual gas reaction. The x-ray rocking curve for the symmetric (0 0 2) and asymmetric (1 0 2) reflections also showed good quality and a small wing tilt of the epitaxial lateral overgrowth (ELO) GaN.

Heavily p-type doped chemical vapor deposition graphene field-effect transistor with current saturation

Current saturation in graphene field-effect transistor (GFET) is of significant importance to improve the maximum oscillation frequency (fmax). We investigated the direct current (dc) and radio frequency (rf) characteristics of a heavily p-type doped GFET based on chemical vapor deposition grown material. The drain current saturation is found in our device. It cannot be explained by the “pinch-off” effect associated with ambipolar transport, but can be attributed to nonlinear channel conductance and velocity saturation in unipolar channel. This study promotes understanding the behaviors of heavily doped GFETs and their radio frequency applications.

Effect of Annealing on the Characteristics of Pd/Au Contacts to p-Type GaN/Al0.45Ga0.55N

The effect of annealing on the characteristics of Pd/Au contacts to p-type GaN/Al0.45Ga0.55N was investigated. The electrical characteristics of Pd/Au contacts and a p-GaN/AlGaN sample were measured after annealing at different temperatures from 550°C to 850°C. Changes in the surface electrical characteristic of p-GaN/AlGaN material were observed after each annealing step. It is indicated that the surface electrical characteristic of p-GaN/AlGaN material plays an important role in the current transport through Pd/Au pads. A possible reason for those changes is impurity contamination, most likely oxygen and carbon contamination introduced from processing. The microstructure of Pd on p-GaN/AlGaN was investigated by glancing-incidence x-ray diffraction (GXRD). The results of GXRD show that the AlPd2 and GaPd2 phases were formed at the interface after annealing, and the formation of these phases could be effective for forming ohmic contacts on p-GaN/AlGaN.

Depressed scattering across grain boundaries in single crystal graphene

We investigated the electrical and quantum properties of single-crystal graphene (SCG) synthesized by chemical vapor deposition (CVD). Quantum Hall effect and Shubnikov de Hass oscillation, a distinguishing feature of a 2-dimensional electronic material system, were observed during the low temperature transport measurements. Decreased scattering from grain boundaries in SCG was proven through extracting information from weak localization theory. Our results facilitate understanding the electrical properties of SCG grown by CVD and its applications in high speed transistor and quantum devices.