STUDENT PROJECTS

All students involved in this overseas experience had the opportunity to work on experimental or computational research projects in the host institutions (the Physics Department of Peking University in 2002 and 2003 and the Semiconductor Institute of the Chinese Academy of Sciences in 1999 and 2001). The students gained first-hand experience and knowledge from diverse fields of physics and had the opportunity to meet and discuss physics with Chinese scientists and their students. All students made good progress on their research projects, and their research reports are shown below.

As a wide band-gap semiconductor gallium nitride promises many electronic and optoelectronic applications. Gallium nitride nanowires are particularly interesting because of their achievable ultra-fine diameters. With a high aspect ratio, these anisotropic crystals are effectively one-dimensional. Since diameters can be smaller than the Bohr radius ( ~11 nm), the nanowires display quantum confinement effects and other properties different from the bulk materials.

In this research, ultrafine GaN nanowires are synthesized from precursors of pure gallium and ammonia gas in a high temperature environment via a Vapor-Liquid-Solid (VLS) process with a modified Chemical Vapor Deposition (CVD) chamber. Results are characterized by SEM (DB235 FIB). To facilitate the experimental work, we have also considered the theory governing the semiconductor nanowire growth, nanowire synthesis by alternative methods and materials, and present and predicted applications of nanoscale materials.

In the semiconductor industry, GaN is important for blue Laser Diodes (LDs) and Light Emitting Diodes (LEDs). In order to maximize efficiency for optoelectronic devices that utilize GaN products, a low contact resistance and an ohmic contact are needed. Previously, the contact resistance has been found to be as low as 10^-4 Ω cm². The aim of this research project was to investigate the influence of different annealing conditions for the contact resistance (ρc); analyze the microstructure of the electrodes; find the relationship between the microstructure, annealing conditions, and ρc; and then explain the mechanism. The sample was grown in a MOCVD system and had a mesa structure. It was activated with Mg-H 800 C for 20 minutes to become a p-type GaN semiconductor. The sample underwent four different annealing conditions. The first condition varied the temperature in constant Oxygen ambient; the second varied the temperature in air; the third varied the percentage of Oxygen with Nitrogen in constant temperature; and the fourth varied the time annealed under Oxygen ambient. The third condition has never previously been tested. We found definite minimums of the contact resistivity (using the TLM method) in the first condition and second conditions at 500 C. The third condition had the best results with a mix of 50% Oxygen and 50% Nitrogen, and the fourth condition had the best results at 5 minutes. Once the effects of the microstructure are analyzed for the sample at each condition, a better understanding of the physical mechanisms to yield the contact resistance will be known.

Regular networks exhibit critical sensitivity in simulations created using the Cluster Mean Field (CMF) model. A similar conclusion about small-world networks cannot yet be made, as attempts at viewing critical sensitivity in a small-world network have not been successful. Further modification of the simulation code is necessary before testing can commence with any certainty.

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Al_xGa_1-xN is a direct band gap semi-conductor that emits radiation in the blue light spectrum thus, allowing for it to have potential applications in optical devices. The band gap of the material can be finely adjusted by altering the Al content in the material. The purpose of this research is to find a relationship between the Al organic flux used during formation of the material and the final Al content present in the material.

Five crystal samples of Al_xGa_1-xN were grown using Metal Organic Chemical Vapor Deposition (MOCVD). The incident Al organic flux used during formation of the crystals, along with the crystal structure and layer thickness varied for each sample. The samples were analyzed using X-ray Diffraction (XRD) and Rutherford Backscattering (RBS) and the Al content in each sample was calculated from these analyses. The XRD results were found to be inaccurate due to the large strain between the varying lattice constants of the structure. Although no obvious trend was found between the Al flux and Al content, some qualitative features of the RBS data suggest that such a trend may exist. The visibility of a clear quantitative trend seems to be hidden by an unexpected Al content dependence on material structure and layer thickness. This has consequently become a topic of our future research.

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The evolution of modern cooperative trends now seen in society are not easily explained thus far. After extensive computational studies and theoretical analysis, Nowak and Sigmund proposed that cooperation was established largely due to the emergence of indirect reciprocity. In previous studies it was found that a high information flow rate stimulates cooperation in a society. In this study we found that the decrease of cooperation cost will make a society more cooperative, and the inheritance of wealth will induce cooperation in the society even when the exchange rate is comparatively low. The distribution of knowledge according to wealth is also studied. We find that for this model, cooperation is slightly less likely to occur if the exchange rate is low.

A variety of gallium oxide nanostructures were synthesized by chemical vapor deposition in a tube furnace using Ni(NO₃)₂ catalyst. Scanning electron microscope analysis showed many one-dimensional nanowires with diameters as low as 30 nanometers and two-dimensional nansheets and nanobelts with thicknesses of 30 to 50 nanometers. Many of the one- and two-dimensional structures appeared to grow from an underlying bed of gallium oxide microparticles. The absence of a single nanoparticle from the tips of many of the nanowires implies that the vapor-liquid-solid (VLS) crystal growth mechanism does not sufficiently explain nanowire growth for gallium oxide. The defect growth mechanism, which describes nanowire growth as induced by defects in the crystal lattice, may provide a better explanation.

When working with a scanning tunneling microscope (STM), you can never expect what will result from its use, due to the fact that it has been and still is the forerunner in topographical views of surfaces. With a resolution of a few square angstroms and vertical less than one angstrom, everything that is seen with the STM is very new. Simply stated, the STM consists of a tip and a sample, both which have to be conducting, inside of a vacuum. With two conducting electrodes with an isolator creating a vacuum gap barrier of about one nanometer, there will be a tunneling effect created. The tunneling will in turn create a current that depends on the separation distance between the electrons, located on the tip and the sample. By manually manipulating the current and the voltage, I was able to create high resolution ‘pictures’ of materials.

Carbon nanotubes (CNTs) have the potential to be used in many applications such as field emitters for displays, proton magnification resonance, nano-scale devices, transistors, H2 storage, new medical technology, and composites. Using CNTs as electron-conducting materials is the first step in application design. The growth mechanism is very important in achieving this goal. The purpose of this project is to successfully create aligned multi-walled CNTs using a hot filament chemical vapor deposition (CVD) furnace.

This report will give a brief overview of the SQUID systems that Peking University has developed and their applications for geophysical survey and manetocardiography (MCG). Then, a detailed process of fabricating superconductive YBCO thin film using the sputtering technique will be presented.

We have measured X-ray diffraction rocking curves (w -scans) of five different faces on five samples of GaN thin films. From the full-width half-maximum (FWHM) of the curves, we have calculated the amount of edge-type and screw-type dislocations in GaN. From previously measured values for carrier mobility, we can learn how the different types of threading dislocations affect carrier mobility. As expected, we have found that higher dislocation densities lead to smaller carrier mobilities. However, our measurements show no clear correlation between carrier mobility and the interdependence parameter m. We also do not find any relationship between the amount of dislocation and the parameter m.

The electronic and optical properties of semiconductors can be tailored to meet specific requirements using engineered materials. The energy band gap configurations of the materials are related to transport, tunneling, optical excitation and recombination processes. Periodic layers of materials with different indexes of refraction can be used as mirrors in a laser cavity. Here GaAs/AlAs superlattices are researched as possible distributed Bragg reflectors (DBRs). The width of the alternating layers has to be an integer multiple of the wavelength of the laser light to be an effective reflector. This is made possible by the atomic length scale control that materials technicians now have. GaAs/AlAs superlattices undergo a transition from type I (direct) semiconductors to type II (indirect) semiconductors as the layer width is decreased below 11 monolayers. This transition was observed experimentally by looking at the relaxation times of several superlattices with different layer widths using time resolved spectroscopy.

GaInNAs is a very useful material because of its large energy gap, allowing us to produce wavelengths of light in the optical fiber transmission window of 1.3 to 1.55mm. Research on such materials have potential to improve optoelectronic devices by helping us understand how things function at the atomic level. This research also runs parallel with improving the Vertical Cavity Surface Emitting Laser (VCSEL), a highly efficient low-cost semiconductor laser.

The research was carried out at the National Laboratory for Superlattices and Microstructures (NLSM) in the Semiconductor Institute of the Chinese Academy of Science (SICAS) in Beijing, China. The experiments were performed in the Fourier Transform Infared Laboratory, under the instruction of Professor De-Sheng Jiang and Graduate Mentor Xiaogan Liang.

As a beginner to the concept of solid state physics and Raman spectroscopy, the professors and grad students were more than willing to help me gain knowledge of the subjects. Having minimal prior knowledge or experience, I still had the opportunities my peers did with hands-on experimentation, use of equipment, etc. I have overcome a great obstacle, that of gaining knowledge of difficult material. At first there is always a question of the possibility that one might conquer the task. It will happen, I have discovered, with effort. This new ability to know that what I want to accomplish can be done, has changed my life and my future plans.

The InAs/AlSb heterojunction system is a lattice matched semiconductor system with type-II band alignment at the interfaces. This system is attractive to ultra-fast electronic device application due to its high mobility and large conduction-band offset. The superlattice can have two different interfaces: AlAs-like or InSb-like. These interfaces have very different interface roughness, mobility and carrier concentrations. There is experimental evidence showing preferential local arrangements of the interfacial atoms. First principle pseudo-potential calculations indicate that such superlattices with switched layers at the interfaces are more energetically favorable than superlattices with ideal sharp strained interfaces. We theoretically calculate the phonon modes of the InAs/AlSb superlattices using a one-dimensional linear chain model. We compare the interface modes for superlattices with the two different interface structures. We found that there not only are more interface modes in the superlattice with the switched interface layers, but that there also exist bulk modes at the interface, a special feature of this structure.

We have developed a computer program from scratch to calculate phonon modes in a three dimensional QDs with up to 64 atoms by using a microscopic valence-force field model (VFFM). This program was run and debugged many times. Finally, our program was tested on Si QDs with a few different sizes, and our results agreed with the existing understanding of quantum confinement effects of phonon modes in QDs.