Haijian Zhong

Charge transport mechanisms of graphene/semiconductor Schottky barriers: A theoretical and experimental study

Graphene has been proposed as a material for semiconductor electronic and optoelectronic devices. Understanding the charge transport mechanisms of graphene/semiconductor Schottky barriers will be crucial for future applications. Here, we report a theoretical model to describe the transport mechanisms at the interface of graphene and semiconductors based on conventional semiconductor Schottky theory and a floating Fermi level of graphene. The contact barrier heights can be estimated through this model and be close to the values obtained from the experiments, which are lower than those of the metal/semiconductor contacts. A detailed analysis reveals that the barrier heights are as the function of the interface separations and dielectric constants, and are influenced by the interfacial states of semiconductors. Our calculations show how this behavior of lowering barrier heights arises from the Fermi level shift of graphene induced by the charge transfer owing to the unique linear electronic structure.

Self-adaptive electronic contact between graphene and semiconductors

Understanding the contact properties of graphene on semiconductors is crucial to improving the performance of graphene optoelectronic devices. Here, we show that when graphene is in contact with a semiconductor, the charge carrier transport into graphene leads to a self-adaptive shift of the Fermi level, which tends to lower the barrier heights of the graphene contact to both n- and p-type semiconductors. A theoretical model is presented to describe the charge carrier transport mechanism and to quantitatively estimate the barrier heights. These results can benefit recent topical approaches for graphene integration in various semiconductor devices.

Modulating microstructure and magnetic properties of BaFe12O19 thin films by using Pt and yttria stabilized zirconia underlayers

The c-axis oriented barium ferrite thin films were prepared by radio frequency magnetron sputtering on silicon substrates with a metal underlayer Pt (111) as well as an oxide underlayer yttria stabilized zirconia (YSZ). Microstructural studies (scanning electron microscopy, atomic force microscopy, and magnetic force microscopy) showed that the magnetic grains in BaM film have a strong relationship with the grains in the underlayer. The Pt underlayer is more effective in forming micrometer-sized and multidomain magnetic grains, which have high saturation magnetization but small coercivity and remanence of the BaM film. On the contrary, the YSZ underlayer is favorable to obtain nanometer-sized and monodomain magnetic grains, which lead to a slight decrease in saturation magnetization but dramatically increase coercivity and remanence of the BaM film. Hence, with careful selection of underlayer, it is feasible to obtain suitable magnetic grain size and domain structure of BaM films to satisfy special requirements.

Constant current etching of gold tips suitable for tip-enhanced Raman spectroscopy.

We introduce a setup and method to produce gold tips that are suitable for tip-enhanced Raman spectroscopy by using a single step constant current electrochemical etch. The etching process is fully automated with only three preset parameters: the etching current, the reference voltage and the immersed length of gold wires. By optimizing these parameters, reproducible high quality tips with smooth surface and a radius curvature of about 20 nm can be formed. Tips prepared with this method were examined by tip-enhanced Raman spectroscopy experiments on the samples of single-wall carbon nanotube, p-aminothiophenol, and graphene. In the Raman mapping of single-wall carbon nanotubes, the spatial resolution is about 15 nm.

Graphene in ohmic contact for both n-GaN and p-GaN

The wrinkles of single layer graphene contacted with either n-GaN or p-GaN were found both forming ohmic contacts investigated by conductive atomic force microscopy. The local I–V results show that some of the graphene wrinkles act as high-conductive channels and exhibiting ohmic behaviors compared with the flat regions with Schottky characteristics. We have studied the effects of the graphene wrinkles using density-functional-theory calculations. It is found that the standing and folded wrinkles with zigzag or armchair directions have a tendency to decrease or increase the local work function, respectively, pushing the local Fermi level towards n- or p-type GaN and thus improving the transport properties. These results can benefit recent topical researches and applications for graphene as electrode material integrated in various semiconductor devices.

Surface acoustic waves in semi-insulating Fe-doped GaN films grown by hydride vapor phase epitaxy

The propagation properties of surface acoustic waves (SAWs) in semi-insulating Fe-doped GaN films grown on sapphire substrates by hydride vapor phase epitaxy are investigated. Compared with native n-type GaN, Fe-doped GaN exhibits a higher electromechanical coupling coefficient due to its high electrical resistivity. In addition, guided longitudinal leaky surface acoustic wave (LLSAW) was observed experimentally with a very high phase velocity (about 7890u2009m/s), and this mode was verified by numerical simulations. The small propagation attenuation of LLSAW along liquid/solid interfaces was demonstrated in glycerol solutions, which implies the potential applications in high-frequency chemical sensing.

Local ultra-violet surface photovoltage spectroscopy of single thread dislocations in gallium nitrides by Kelvin probe force microscopy

The local carrier properties, including minority diffusion lengths and surface recombination velocities, were measured at single thread dislocations in GaN film by a combination of surface photovoltage spectroscopy and Kelvin probe force microscopy. The thread dislocations introduced by a nanoindentation were observed as V-pits, where the photovoltage was lower than that on plane surface under ultra-violet illumination. A model is proposed to fit the spatially resolved surface photovoltage spectroscopy curves. Compared with those on plane surface, the hole diffusion length is 90 nm shorter and the surface electron recombination velocity is 1.6 times higher at an individual thread dislocation.

Structure, Stress State and Piezoelectric Property of GaN Nanopyramid Arrays

GaN nanopyramid (NP) arrays have been fabricated by a convenient electrodeless photoelectrochemical etching method. Transmission electron microscopy measurement indicates that these NPs are composed of crystalline GaN surrounding a dislocation. High-resolution X-ray diffraction and the micro-Raman spectrum reveal a highly compressive stress relaxation in the NPs compared with compressed GaN subfilm. Additionally, negative piezoelectric current pluses are generated from the GaN NPs when the conductive atomic force microscope scans cross the arrays in the contact mode. The result demonstrates that the GaN NP arrays are a promising candidate for nanogenerators.