Senlin Li

Low defect concentration few-layer graphene using a two-step electrochemical exfoliation

Low defect concentration few-layer graphene (FLG) sheets were fabricated by a two-step electrochemical intercalation exfoliation, including a graphite foil pretreatment in sodium hydroxide solution and a subsequent further exfoliation in sulfuric acid solution. During the process, the pretreatment results in the expansion of the graphite foil and in turn facilitates the final exfoliation in sulfuric acid solution. The results show that the I(D)/I(G) of the obtained FLG sheets is as low as 0.29 while maintaining relatively high yield, more than 56%. In addition, the oxygen content in the FLG sheets is 8.32% with the C/O ratio of 11.02.

The Effect of AlN Nucleation Temperature on the Growth of AlN Films via Metalorganic Chemical Vapor Deposition

AlN epilayers were grown directly on sapphire (0001) substrates using a combined growth scheme, consisting of a low-temperature nucleation layer and a second layer grown by high-temperature pulsed atomic layer epitaxy via metalorganic chemical vapor deposition. With an emphasis on the nucleation layer, its growth temperature was varied from 470°C to 870°C, and obvious differences in the surface morphology, crystal quality, and strain states of the overall AlN epilayers were observed. Based on atomic force microscopy, x-ray diffraction, and Raman spectroscopy results, these differences are ascribed to the nucleation sites and the subsequent grain size. Due to the enhanced mobility of Al adatoms with increasing temperature, the nucleation sites decrease and the subsequent grain size increases, leading to the achievement of atomically flat AlN epilayers with good crystal quality for the nucleation layer grown at 570°C. However, at higher nucleation layer growth temperature, the properties of the AlN epilayers deteriorate due to the possible appearance of misaligned AlN grains. A model is also developed according to all observations.

Effect of the Al0.3Ga0.7As interlayer thickness upon the quality of GaAs on a Ge substrate grown by metal-organic chemical vapor deposition

GaAs epilayers on Ge substrates are grown with a thin Al0.3Ga0.7As interlayer via metal-organic chemical vapor deposition with the goal of investigating the effect of the Al0.3Ga0.7As interlayer thickness upon the GaAs epilayer. The results show that as the Al0.3Ga0.7As interlayer thickness increases from 0 to 30 nm, both the crystal quality and surface morphology of the GaAs epilayer follow a trend of melioration and then deterioration. All of the Al0.3Ga0.7As interlayers investigated are seen to effectively block the diffusion of Ge atoms to the GaAs epilayers, and high crystalline quality GaAs epilayers with a smooth surface are obtained by growing a 15–23 nm-thick Al0.3Ga0.7As interlayer.

Investigation of GaInAs strain reducing layer combined with InAs quantum dots embedded in Ga(In)As subcell of triple junction GaInP/Ga(In)As/Ge solar cell

The InAs/GaAs quantum dots structure embedded in GaInP/Ga(In)As/Ge triple junction solar cell with and without Ga0.90In0.10As strain reducing layer was investigated. Conversion efficiency of 33.91% at 1,000 suns AM 1.5D with Ga0.90In0.10As strain reducing layer was demonstrated. A 1.19% improvement of the conversion efficiency was obtained via inserting the Ga0.90In0.10As strain reducing layer. The main contribution of this improvement was from the increase of the short-circuit current, which is caused by the reduction of the Shockley–Read–Hall recombination centers. Consequently, there was a decrease in open circuit voltage due to the lower thermal activation energy of confined carriers in Ga0.9In0.1As than GaAs and a reduction in the effective band gap of quantum dots.

Large-scale growth of density-tunable aligned ZnO nanorods arrays on GaN QDs

An effective approach for growing large-scale, uniformly aligned ZnO nanorods arrays is demonstrated. The synthesis uses a GaN quantum dot (QD) template produced by a self-assembled Stranski-Krastanow mode in metal organic chemical vapor deposition, which serves as a nucleation site for ZnO owing to the QDs high surface free energy. The resultant ZnO nanorods with uniform shape and length align vertically on the template, while their density is easily tunable by adjusting the density of GaN QDs, which can be adjusted by simply varying growth interruption. By controlling the density of ZnO nanorod arrays, their optical performance can also be improved. This approach opens the possibility of combining one-dimensional (1D) with 0D nanostructures for applications in sensor arrays, piezoelectric antenna arrays, optoelectronic devices, and interconnects.

Defect Reduction in AlN Epilayers Grown by MOCVD via Intermediate-Temperature Interlayers

In this work, significant reduction of the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates via metalorganic chemical vapor deposition has been obtained by insertion of thin intermediate-temperature interlayers (IT-ILs). The growth temperature of the IT-ILs ranged from 750°C to 950°C after the initial growth at high temperature of 1200°C. Detailed characterizations were performed to understand the mechanisms of the reduction in dislocation density. It is found that the relatively low growth temperature of the IT-ILs can modify the originally two-dimensional (2D) growth mode to a three-dimensional (3D) growth process and creates a high density of small islands at the interface. During the subsequent growth of the high-temperature AlN layer, the AlN islands initially coalesce to form larger grains as the growth proceeds, and some TDs present in the previous AlN epilayers are found to bend near the interlayer, leading to dislocation merging and annihilation.

Defect reduction in Si-doped Al0.45Ga0.55N films by SiNx interlayer method

The dislocation density in AlGaN epitaxial layers with Al content as high as 45% grown on sapphire substrates has been effectively reduced by introducing an in-situ deposited SiNx nanomask layer in this study. By closely monitoring the evolution of numerous material properties, such as surface morphology, dislocation density, photoluminescence, strain states, and electron mobility of the Si-Al0.45Ga0.55N layers as the functions of SiNx interlayer growth time, the surface coverage fraction of SiNx is found to be a crucial factor determining the strain states and dislocation density. The dependence of the strain states and the dislocation density on the surface coverage fraction of SiNx nanomask supports the very different growth models of Al-rich AlGaN on SiNx interlayer due to the reduced nucleation selectivity compared with the GaN counterpart. Compared with GaN, which can only nucleate at open pores of SiNx nanomask, Al-rich AlGaN can simultaneously nucleate at both open pores and SiNx covered areas. Di...

InAs/GaAs quantum dots with wide-range tunable densities by simply varying V/III ratio using metal-organic chemical vapor deposition

The complicated behaviors of InAs/GaAs quantum dots with increasing V/III ratio associated with several competing mechanisms have been described. The results demonstrate that the densities of InAs quantum dots can be tuned in a wide range from 105 to 1010 cm−2 by simply manipulating V/III ratio via metal-organic chemical vapor deposition. These results are mainly ascribed to the changes of coverage and In adatom migration length due to the increasing V/III ratio.

Fabrication of GaN nanodots via GaN thermal decomposition in H2 atmosphere

GaN nanodots were fabricated by thermal decomposition of GaN in H2 atmosphere at high temperatures. By varying annealing time and temperature, it was found that dot size and density were highly dependent on annealing conditions. Surface morphology of the sample indicates that the dot formation mechanism is chemical erosion under H2 flow. The etching proceeds in lateral directions, and the dots are formed in hexagonal pit areas. The fitted activation energy of GaN decomposition agrees well with the chemical properties of GaN in the experimental conditions.

Synthesis of ZnO nanowire array film on Mg doped gallium nitride substrate

ZnO nanowire array films, composed of well aligned ZnO nanowires ∼200 nm in diameter and 1 μm in length, were successfully synthesised on Mg doped gallium nitride by hydrothermal method. In addition, the films possess quite flatten surface. In the synthesised process, there was no catalyst that had been used. Growth conditions were comprehensively discussed in the process of aqueous solution method. It was found that the length of ZnO nanowires and the thickness of the film could be tunable by altering solution concentration and growth time. Such ZnO film assembled with vertically aligned nanowire may have potential applications as UV light emitting diodes.